Filter out plainJSErrors

[jakebailey]

Fixes #928

This really nerd sniped me, oops.

This isn't exactly the same; missing the idea of a "partial" check, and we do JSDoc differently, but it seems correct enough. Didn't know that we did this filtering.

[Copilot]

Pull Request Overview

This PR introduces filtering of semantic diagnostics for plain JavaScript files so that only a whitelist of allowed error codes is reported.

• Moves the initial collection of diagnostics earlier in getSemanticDiagnosticsForFile to apply filtering before other checks.
• Adds a plainJSErrors set and applies it to filter diagnostics when IsPlainJSFile is true.
• Introduces IsPlainJSFile helper and updates related utilities; updates test baselines to remove now-filtered JS errors.

Reviewed Changes

Copilot reviewed 117 out of 117 changed files in this pull request and generated 3 comments.

File	Description
testdata/baselines/reference/...errors.txt	Removed all plain-JS-specific error baselines after filtering.
internal/compiler/program.go	Moved diagnostics collection and added plain-JS filtering.
internal/checker/utilities.go	Replaced local isPlainJSFile with call to new helper.
internal/ast/utilities.go	Added IsPlainJSFile helper function.

Comments suppressed due to low confidence (1)

internal/compiler/program.go:811

• [nitpick] The identifier plainJSErrors could be misleading. Consider renaming it to something like allowedPlainJSDiagnostics or plainJSWhitelist to clarify that it's a whitelist of diagnostics to keep.

var plainJSErrors = core.NewSetFromItems(

[jakebailey]

No clue why these errors are dropped

[sandersn]

Thanks for getting to this. I'm a little sad to lose some errors in JS tests while I'm working on jsdoc but that's what type baselines are for.

Probably should delete that dupe before merging.

[jakebailey]

Will need another review now that this repo is seen as a production repo.