Create jsii/utils module

## :rocket: Feature Request

There is a lot of shared code between the different jsii tools (finding/loading assemblies, reading package.json, some common interactions with the TypeScript compiler...).

These are currently duplicated--they should be unified into a single utils package.

Start by surveying the duplicated code in the jsii tools for finding and loading assemblies, reading package.json, and interacting with the TypeScript compiler. Done means the shared behavior is unified in a single utils package rather than duplicated across the tools.

Written by the indexing model from the issue text.