make local build callable

[annajowang]

move main adapter build function to index.ts.

In the future we probably want to move some implementation details back into some internal file (not included in tsconfig.json).

[Yuangwang]

Just one question - how does this make local build callable?

[annajowang]

Just one question - how does this make local build callable?
it makes adapterbuild() directly importable / callable by the firebase CLI rather than making the firebase CLI call npm exec apphosting-local-build
index.ts is part of the "include" of tsconfig.js.
adapter-builds.ts is not. considered adding adapter-build.ts to the include list. but i think it's preferable to keep that list short and not include everything, later when more impelmentation logic / details is added, we should do them through helper functions not directly in the index.ts file