preserve imports from stale files

[m-Bilal]

This PR adds support for automatically preserving import statements in functions.ts and api.ts files

Why

This was required because if a user had a setup in which they were using functions from files other than api.ts and had @save annotation on those functions, re-running the codegen (introspect with overwrite set to true) would preserve the function, but would lose its import statement. This meant that the user would have to re-write their import statements again, which a really bad experience.

Note

import statements are always preserved and organized. They don't need a @save annotation.

Review

Since this is a relatively large PR, it'll be easier to review it per commit