Port more resolveExternalModule, fix copy paste error in GetResolutionDiagnostic

[jakebailey]

This fixes this error from the VS Code repo:

node_modules/@vscode/vscode-languagedetection/dist/lib/index.d.ts:1:8 - error TS2307: Cannot find module '@tensorflow/tfjs-backend-cpu' or its corresponding type declarations.

1 import '@tensorflow/tfjs-backend-cpu';
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Found 1 error in node_modules/@vscode/vscode-languagedetection/dist/lib/index.d.ts:1

Among other problems.

[Copilot]

Pull Request Overview

This PR centralizes module resolution diagnostics into GetResolutionDiagnostic, removes a duplicate implementation in fileloader.go, and updates error reporting in the checker to emit extension-related diagnostics correctly.

• Introduces GetResolutionDiagnostic in internal/module/util.go to handle JSX, JS, JSON, and arbitrary-extension rules.
• Refactors fileloader.go to call the new centralized diagnostic function and removes the old local copy.
• Enhances checker.go to capture and report the new diagnostics early in external module resolution.

Reviewed Changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated no comments.

File	Description
internal/module/util.go	New GetResolutionDiagnostic function for extension-based errors.
internal/compiler/fileloader.go	Replaced local diagnostic call with module.GetResolutionDiagnostic and removed duplicate logic.
internal/checker/checker.go	Added resolutionDiagnostic handling and updated source-file/error logic, plus helper resolutionExtensionIsTSOrJson.

Comments suppressed due to low confidence (4)

internal/module/util.go:135

• New branches in GetResolutionDiagnostic (especially the default case for arbitrary extensions) should have dedicated unit tests to ensure correct diagnostics are emitted for each extension type.

	switch resolvedModule.Extension {

[DanielRosenwasser]

I'll admit I don't know how/why this fixes the error in VS Code (maybe you can comment on why), but the implementation looks like it matches the existing Strada behavior.

[DanielRosenwasser]

This is in the current snapshot, but does this not kick in for node20 in Strada?

[jakebailey]

See the description on microsoft/TypeScript#61805; it would if Corsa had node20.

[jakebailey]

I'll admit I don't know how/why this fixes the error in VS Code (maybe you can comment on why), but the implementation looks like it matches the existing Strada behavior.

The VS Code problem is specifically that a side effect import was being resolved to a JS file (lacking type definitions), but we weren't handling that case.