[Feature]: supports custom mainField

[hardfist]

I know it's kind of out of scope, but it would be really nice if tsm could supports custom mainField, then we can do something like this.

• xxx-lib/package.json

{
  "source": "src/index.ts",
  "main": "dist/index.js",
  "types": "dist/index.d.ts"
}

we could use tsm to load the xxx-lib source file without compile it to js in monorepo.