Set context keys for cell language when background is painted

[juliasilge]

This is a revamp of the work in #607.

Unfortunately after I started trying to use this in Positron, it just wasn't early enough or quick enough to set these context keys in languageAtPosition(). I believe that only gets called if we are trying to do one of these:

quarto/apps/vscode/src/vdoc/vdoc.ts

Lines 114 to 121 in d9d1515

	export type VirtualDocAction =
	"completion" |
	"hover" |
	"signature" |
	"definition" |
	"format" |
	"statementRange" |
	"helpTopic";

It's just not enough, especially if you are switching back and forth between different files; it's very easy to get in a state where you end up with R shortcuts in Python chunks and there are lots of kinds of edits I could make that did not trigger one of these.

New idea! This PR tries out setting these context keys when we paint the background. In my experience so far, this feels a lot better, and we already are identifying all the tokens so not much extra work.

[juliasilge]

Related to posit-dev/positron#5451