Migrate PLUGS to space config

[zefhemel]

Having a separate PLUGS file for maintaining what plugs to activate made sense at the time. But by now we have a pretty robust alternative with space config and should just move to that.

This can be done in a backwards compatible way by supporting both, with space config becoming the new way.

To figure out: how to implement Plugs: Add, which currently updates your PLUGS file automatically.

[nodu]

Perhaps drop the Plugs: Add command... ?

[Maarrk]

Proposal:

1. Keep the plugs list in Space Config as:

```space-config
plugs:
- github:silverbulletmd/silverbullet-git/git.plug.js
- ghr:Maarrk/silverbullet-grep
```

2. Plugs: Add appends to this list on the current page on the PLUGS page

• if there is no space-config block, add it at the end of the page
• if the space-config block doesn't have plugs: at the top level, append it at the end of the block
• append the URI into the list

3. If the PLUGS page contains the yaml block with a list, it changes it into the space-config block with the list under plugs: - this way the update happens automatically for the user, but in front of their eyes

---

Potential problem: there is another way to write a list in YAML:

```space-config
plugs: [ github:silverbulletmd/silverbullet-git/git.plug.js, ghr:Maarrk/silverbullet-grep ]
```

But I think this can also be handled reasonably if the inserting is done on the YAML as a parsetree instead of just adding a line

[zefhemel]

I think this makes sense. The only nasty thing is you probably want to update YAML without rewriting it (e.g. so that comments get lost, reformat everything). For the PLUGS file the whole plug list was always rewritten when using Plugs: Add I think, but in the new system I wouldn't want to happen. It should somehow attempt a minimal change.

[Maarrk]

New idea: the Plugs: Add command accepts a string argument that fills the editor.prompt, there can be a button {[Plugs: Add|Install now](https://example.com/cool.plug.js)}. The prompt would still show the user the address before adding it, so I think it's OK

Because of that, I'm back to the idea that the command should edit the PLUGS page, even if they can be loaded from anywhere, updated the #1042 (comment)

[Maarrk]

Finally got around to finishing this in #1122

For the PLUGS file the whole plug list was always rewritten when using Plugs: Add I think, but in the new system I wouldn't want to happen. It should somehow attempt a minimal change.

This behaviour is separated into insertIntoPlugPage function, which got a bit large in order to support all the cases with minimal edits, but at least it's well commented (I think) and has tests written for a bunch of scenarios.

[Maarrk]