github-slugger support and markdown-it-wikilinks rendering in preview

Enhancements:

• merge #85 - add setting vscodeMarkdownNotes.slugifyMethod - "classic" supports some options specific to Markdown Notes. "github-slugger" uses https://github.com/Flet/github-slugger and is compatible with foam, addresses: foambubble/foam#240
• merge #87 - add API.ts with a 'beta' public API (subject to change), one example function:

let notes = await vscode.commands.executeCommand('vscodeMarkdownNotes.notesForWikiLink', 'demo');

• add a sync version of filesForWikiLinkRef, filesForWikiLinkRefFromCache
• merge #86 from @thomaskoppelaar (🙏🏿 🙏🏿 🙏🏿), with...
• markdownItPlugins rendering of wiki-links
• add setting: vscodeMarkdownNotes.previewShowFileExtension - Specifies whether or not to show the linked file's extension in the preview
• add setting: vscodeMarkdownNotes.previewLabelStyling - Changes how the link to a file should be shown, either with or without brackets

Fixes:

• (#88) fix bug with jest-focused.sh test runner and fix test-at-line.ts so it can parse typescript files

Cleanup:

• cleanup jest tests
• add setConfig helper to jest tests

Diff:

https://github.com/kortina/vscode-markdown-notes/compare/8efafabe9d299a7e2c1d34559764952aa338888c..4326868eca031f52d79b0d779347daadc57bbaea