This commit splits the ngtsc `core` package's api entrypoint, which
previously was a single `api.ts` file, into an api/ directory with multiple
files. This is done to isolate the parts of the API definitions pertaining
to the public-facing `angularCompilerOptions` field in tsconfig.json into a
single file, which will enable a public API guard test to be added in a
future commit.
PR Close#35885