Notebooks/basic html output support (#4058)

Addresses #4018 

- Add basic support for HTML output, aka things like pandas dataframe
tables.
- Only show a single output element per cell output. 
- A cell output can have multiple data bundles of `mime` + `data`,
previously we would render all of them. Now we just choose one to
render, like other notebook renderers.
- Currently, this is done via a fairly arbitrary ordering of
"interesting" output types, but in the future we may be more structured
about it.

<img width="1199" alt="image"
src="https://github.com/user-attachments/assets/3e9b7e89-855c-48c0-82a3-dcf6ebe4b823">

Also includes a large shuffling of the positron instance code to better
organize it according to how other code in positron is organized with
private and public fields separated.


### QA Notes

Notebooks should continue to work as normal, just simple html outputs
will now actually render. E.g. the above table. Here's the code to
generate one:
```python
import pandas as pd
import numpy as np

# Generate random data
pd.DataFrame({
    'A': np.random.randint(0, 100, size=20),
    'B': np.random.normal(0, 1, size=20),
    'C': np.random.choice(['X', 'Y', 'Z'], size=20),
    'D': np.random.uniform(0, 1, size=20)
})
```

src/vs/workbench/contrib/positronNotebook/browser/PositronNotebookCell.ts

@@ -11,7 +11,7 @@ import { ITextModelService } from 'vs/editor/common/services/resolverService';
 import { IInstantiationService } from 'vs/platform/instantiation/common/instantiation';
 import { NotebookCellTextModel } from 'vs/workbench/contrib/notebook/common/model/notebookCellTextModel';
 import { CellKind } from 'vs/workbench/contrib/notebook/common/notebookCommon';
-import { ExecutionStatus, IPositronNotebookCodeCell, IPositronNotebookCell, IPositronNotebookMarkdownCell, NotebookCellOutputs } from 'vs/workbench/services/positronNotebook/browser/IPositronNotebookCell';
+import { ExecutionStatus, IPositronNotebookCodeCell, IPositronNotebookCell, IPositronNotebookMarkdownCell, NotebookCellOutputs, NotebookCellOutputItem } from 'vs/workbench/services/positronNotebook/browser/IPositronNotebookCell';
 import { CodeEditorWidget } from 'vs/editor/browser/widget/codeEditor/codeEditorWidget';
 import { CellSelectionType } from 'vs/workbench/services/positronNotebook/browser/selectionMachine';
 import { PositronNotebookInstance } from 'vs/workbench/contrib/positronNotebook/browser/PositronNotebookInstance';
@@ -24,15 +24,12 @@ export abstract class PositronNotebookCellGeneral extends Disposable implements
 	private _container: HTMLElement | undefined;
 	private _editor: CodeEditorWidget | undefined;
 
-
-
 	constructor(
 		public cellModel: NotebookCellTextModel,
 		public _instance: PositronNotebookInstance,
 		@ITextModelService private readonly textModelResolverService: ITextModelService,
 	) {
 		super();
-
 	}
 
 	get uri(): URI {
@@ -219,4 +216,64 @@ export function createNotebookCell(cell: NotebookCellTextModel, instance: Positr
 }
 
 
+/**
+ * Get the priority of a mime type for sorting purposes
+ * @param mime The mime type to get the priority of
+ * @returns A number representing the priority of the mime type. Lower numbers are higher priority.
+ */
+function getMimeTypePriority(mime: string): number | null {
+	if (mime.includes('application')) {
+		return 1;
+	}
 
+	switch (mime) {
+		case 'text/html':
+			return 2;
+		case 'image/png':
+			return 3;
+		case 'text/plain':
+			return 4;
+		default:
+			// Dont know what this is, so mark it as special so we know something went wrong
+			return null;
+	}
+}
+
+/**
+ * Pick the output item with the highest priority mime type from a cell output object
+ * @param outputItems Array of outputs items data from a cell output object
+ * @returns The output item with the highest priority mime type. If there's a tie, the first one is
+ * returned. If there's an unknown mime type we defer to ones we do know about.
+ */
+export function pickPreferredOutputItem(outputItems: NotebookCellOutputItem[], logWarning: (msg: string) => void): NotebookCellOutputItem | undefined {
+
+	if (outputItems.length === 0) {
+		return undefined;
+	}
+
+	let highestPriority: number | null = null;
+	let preferredOutput = outputItems[0];
+
+	for (const item of outputItems) {
+		const priority = getMimeTypePriority(item.mime);
+
+		// If we don't know how to render any of the mime types, we'll return the first one and hope
+		// for the best!
+		if (priority === null) {
+			continue;
+		}
+
+		if (priority < (highestPriority ?? Infinity)) {
+			preferredOutput = item;
+			highestPriority = priority;
+		}
+	}
+
+	if (highestPriority === null) {
+		logWarning('Could not determine preferred output for notebook cell with mime types' +
+			outputItems.map(item => item.mime).join(', ')
+		);
+	}
+
+	return preferredOutput;
+}