Recognize .mts and .cts TypeScript modules

[amic25]

Goal

Treat modern .mts and .cts module files as TypeScript sources.

Scope

• Update source discovery/language mapping.
• Add positive discovery tests for both extensions.
• Add an exclusion regression test.
• Update the supported-file documentation if needed.

Acceptance criteria

• Both extensions are scanned as TypeScript.
• Existing exclusion and size limits still apply.
• make check passes.