editorService & modelService refactor and optimizations

Created by: lguychard

Still a fair bit of cleanup to do here, but this introduces some significant improvements towards https://github.com/sourcegraph/sourcegraph/issues/5172.

Notable changes:

1. Refactor editorService, modelService to be backed by Maps, rather than immutable arrays.
2. Emit incremental updates of added/updated/deleted editors and models, rather than a new array of all editors/models on every emission. This dramatically reduces GC pressure, and improves efficiency of adding/updating/deleting models and editors.
3. Introduces editorService.activeEditorUpdates, to avoid needless iteration over all editors when computing context keys.
4. Introduces editorService.activeLanguages (still very rough, may move to modelService) to avoid needless iteration over all editors when computing active extensions.

All tests passed before I started working on 3. and 4.

On the 999-file PR (https://github.com/sourcegraph/sourcegraph/pull/5157/files#diff-1de4be34265b5e830d0e2a2fea7f386bR866), I was able to reliably load up to 400 code views, including blob fetching, in <90seconds. On the same PR, an extension built from master stalls after 270 code views loaded in ~5 minutes.

Created by: lguychard

Should have opened as a draft PR, not quite ready for thorough review, but I'll happily take feedback on the general direction.

Created by: codecov[bot]

Codecov Report

Merging #5464 into master will decrease coverage by 0.72%. The diff coverage is 78.77%.

@@            Coverage Diff             @@
##           master    #5464      +/-   ##
==========================================
- Coverage   47.79%   47.07%   -0.73%     
==========================================
  Files         752      742      -10     
  Lines       46170    45487     -683     
  Branches     2720     2623      -97     
==========================================
- Hits        22068    21414     -654     
+ Misses      22036    22007      -29     
  Partials     2066     2066

Impacted Files	Coverage Δ
browser/src/libs/code_intelligence/text_fields.tsx	84.84% <ø> (ø)
shared/src/api/client/services.ts	100% <ø> (ø)
shared/src/extensions/controller.ts	15.87% <0%> (ø)
web/src/repo/blob/discussions/mentionCompletion.ts	40% <0%> (+11.42%)
shared/src/api/client/context/context.ts	96.66% <100%> (ø)
...src/components/editorTextField/EditorTextField.tsx	61.81% <100%> (ø)
shared/src/api/client/services/location.ts	95.45% <100%> (-0.38%)
shared/src/api/integration-test/testHelpers.ts	92.3% <100%> (+0.2%)
shared/src/api/client/services/editorService.ts	100% <100%> (+3.44%)
shared/src/api/extension/api/textDocument.ts	72.41% <37.5%> (-3.54%)
... and 20 more

Created by: lguychard

Quick plot of loading 400 code views (including resolving revs / fetching file contents) on this branch vs. master:

• The extension built from refactor-editors-models loaded 400 code views in 49.2 seconds.
• The extension built from master crashed the page after loading 370 code views in a little over 8 minutes.

We'll likely still need to look into using IntersectionObserver to lazily load code views, as blocking interactions with the page for ~40 seconds is not acceptable, even on very large PRs, but at least editor/model updates are reasonably efficient now.

All tests pass, I'll do another pass on the code to improve docstrings and other warts, but the general structure is unlikely to change at this stage.

Created by: lguychard

@felixfbecker this is ready for review. As per our conversation yesterday, I stopped including PartialModel in editors by default, the model now always must be looked up in the ModelService.