feat: experimental support for splitting code by tags / route segments

[mnahkies]

introduces a new experimental option --grouping-strategy used by the typescript-koa
template to split the routes into separate files, based on either their first tag or route segment.

includes a lot of refactoring to support this:

• ImportBuilder now supports relative paths in different directories
• TypeBuilder / SchemaBuilder now gets an ImportBuilder scoped to the collection of routes being built for a given output file
• Logic around generating the raw file content for output is now consolidated, and supports merging separate units together prior
• Improved the unit tests for JoiSchemaBuilder and fixed a few bugs in the process
• Fixed the lint-staged command for eslint
• Replaced the jest reporter with one that's a bit quieter (still not perfect)

there's still quite a bit of impedance mismatch that makes outputting in different ways like this more
challenging than it would ideally be, mainly that we go straight from IRModel -> string and quickly
loose context of that string (eg: what imports it depends on, where it is used)

eventually we'll probably need to introduce some wrapping object, similar to the CompilationUnit
introduced in this change that can track interesting things and allow us to be smarter about how
we organize the resulting files.