Support .ts module imports in TypeSpec source files

[Copilot]

TypeSpec imports currently recognize .tsp, .js, and .mjs, which prevents directly importing TypeScript decorator modules. This change adds first-class .ts import recognition so TypeSpec routes those imports through the existing JS module loading path.

• Compiler import classification

  • Updated getSourceFileKindFromExt to classify .ts as "js" in packages/compiler/src/core/source-file.ts.
  • This enables import "./decorators.ts"; to flow through the same runtime import path as existing JS imports.

• Import checker coverage

  • Added .ts import tests in packages/compiler/test/checker/imports.test.ts, mirroring existing .js coverage:
    • relative .ts import
    • parent-folder relative .ts import

• Compiler e2e coverage

  • Added an end-to-end scenario in packages/compiler/test/e2e/scenarios/import-relative-ts/ and wired it into packages/compiler/test/e2e/scenarios/scenarios.e2e.ts.
  • The scenario verifies a TypeSpec source importing ./decorators.ts compiles successfully.

• Language docs

  • Updated website/src/content/docs/docs/language-basics/imports.md to list .ts as a supported explicit file extension for imports.

import "./my-decorators.ts";

@myDecorator
model A {}

[timotheeguerin]

@copilot can we have one e2e test here also?

[Copilot]

@copilot can we have one e2e test here also?

Added an e2e scenario coverage for this in b35f639e (import-relative-ts + scenarios.e2e.ts).

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commit: 80f4f9f