Expose config loading helpers

`loadTypeSpecConfig` is not accessible from the compiler.

This should really only be useful if a tool wants to do additional validation on the config. A user that wants to call the compiler programtically can already call `resolveCompilerOptions()` and this will resolve the compiler options using the config. But we hide the internal of hte config resolution.

We should probably scope that under another export than the root one like `@typespec/compiler/tsp-config`