[TS] Fix relative import paths of generated TypeScript code

[auschi-shieldai]

This PR fixes two issues:

• Resolves an issue where relative import paths are not generated correctly for dependents with no namespace
• Refactors how relative imports are generated, removing the need to back out multiple, unnecessary directory levels

A new test suite has been added to validate relative import paths, as well as the fix itself.

This PR also sneaks a couple linting/formatting fixes in for unrelated parts of the idl_gen_ts.cp file.

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[bjornharrtell]

Looks good to me!