Make `.esbuild` file a module

This prevents this annoying warning from getting printed all the time

```
[watch-copilot         ] [watch:esbuild  ] (node:12319) [MODULE_TYPELESS_PACKAGE_JSON] Warning: Module type of file:///Users/matb/projects/vscode/extensions/copilot/.esbuild.ts is not specified and it doesn't parse as CommonJS.
[watch-copilot         ] [watch:esbuild  ] Reparsing as ES module because module syntax was detected. This incurs a performance overhead.
[watch-copilot         ] [watch:esbuild  ] To eliminate this warning, add "type": "module" to /Users/matb/projects/vscode/extensions/copilot/package.json.
[watch-copilot         ] [watch:esbuild  ] (Use `node --trace-warnings ...` to show where the warning was created)
```

Co-authored-by: Copilot <copilot@github.com>