Implement auto-import code actions, port tests and fix some bugs

[jakebailey]

Fixes #1658

This is not perfect. It passes maybe 60 out of 160 ported fourslash tests so far. Many issues exist with import sorting or formatting, and there are other diagnostics fixed by import fixes that are not being addressed here (like swapping a non-type to a type), and there are other auto-imports that seem weird that I need to double check.

But for the typical cases of just missing an import, this seems to work.

Re-ported a bunch of code and it's now passing tests except these differences:

• symlink tests don't work
• import ordering sometimes differs
• a bit of formatting weirdness in some tests
• JSDoc @import doesn't work (we see the reparsed nodes...)

[Copilot]

Pull Request Overview

This PR implements auto-import fix functionality (code actions) for the TypeScript language server port. It adds support for generating quick-fix code actions that automatically add missing import statements when identifiers cannot be found.

Key changes:

• New code actions handler that detects import-related diagnostics and suggests fixes
• LSP server integration for the textDocument/codeAction request
• Enhanced test infrastructure to support fourslash tests with implicit first files
• Utility function to check symbol meaning compatibility

Reviewed Changes

Copilot reviewed 170 out of 170 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/ls/codeactions.go	New file implementing code action provider with auto-import fix logic
internal/lsp/server.go	Registers code action handler and advertises capability to clients
internal/ls/utilities.go	Adds symbolFlagsHaveMeaning helper for filtering exports by semantic meaning
internal/testrunner/test_case_parser.go	Adds support for implicit first file in fourslash tests
internal/fourslash/fourslash.go	Implements VerifyImportFixAtPosition test method
internal/fourslash/test_parser.go	Updates parser to use new implicit first file option
internal/fourslash/_scripts/convertFourslash.mts	Adds conversion support for importFixAtPosition verification
internal/fourslash/_scripts/failingTests.txt	Tracks test status (many still failing as noted in PR description)
Multiple test files	Adds 160+ fourslash tests for import fix scenarios

[jakebailey]

I'm currently reworking this with less of a minimal implementation that passes way more tests, which will fix all of the CI problems too.