feat: support typespec as an input format

[mnahkies]

adds https://typespec.io/ as a supported input format, using it's typespec -> openapi3 conversion package.

• new optional cli option --input-type=[openapi3|typespec], defaulting to openapi3
• converts provided typespec files to openapi3 on the fly, and then proceeds with generate as normal
• it could probably be better, I haven't figured out what to do with multiple versions / services in a single spec
• requires that you manually install the relevant optional peerDependencies, eg:

yarn add "@typespec/compiler" "@typespec/http" "@typespec/openapi" "@typespec/openapi3" "@typespec/versioning"

also finally got sick of xargs requiring a 255 exit code to halt on failure, and switched the integration test generation scripts over to node, which also prefixes the output to make it easier to follow.

resolves #148

[mnahkies]

These are just required by the integration tests, hence not included in peerDependencies

[mnahkies]

Split from packages/openapi-code-generator/src/core/file-loader.ts - not actually changed, aside from using json5 for parsing json input

[mnahkies]

Split from packages/openapi-code-generator/src/core/file-loader.ts - not actually changed

[mnahkies]

This is the meat of the implementation