JS Plugin Hooks for the Code Editor

[asg017]

When #2052 merges, I'd like to add support to add extensions/functions to the Datasette code editor.

I'd eventually like to build a JS plugin for sqlite-docs, to add things like:

• Inline documentation for tables/columns on hover
• Inline docs for custom functions that are loaded in
• More detailed autocomplete for tables/columns/functions

I did some hacking to see what this would look like, see here:

There can be a new hook that allows JS plugins to add new "extension" in the CodeMirror editorview here:

datasette/datasette/static/cm-editor-6.0.1.js

Line 25 in 8cd60fd

extensions: [

Will need some more planning. For example, the Codemirror bundle in Datasette has functions that we could re-export for plugins to use (so we don't load 2 version of "@codemirror/autocomplete", for example.