[WIP] Fix usage of string literals in esbuild

[Copilot]

• Understand the issue: import foo = "string" should be allowed (like esbuild does)
• Update parser to accept string literals in parse_ts_import_equals_declaration
• Update transformer to handle string literal module references
• Add tests for the new functionality
• Run tests to verify changes

Original prompt

Please fix #16675 (comment), since esbuild even allows string literals. See evanw/esbuild@2102e06#diff-77afe5d24e480c64dcd9bc0ae8b6d8849788be2ab5c28442929ce002be9420e2R1186-R1208

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.