Support for an src/api.ts file

[campbellman]

Summary

This adds support for a new optional file (default api.ts) where developers can add custom middleware, non-react endpoints, and define a custom handler based on their deployment needs.

Motivations

• The current example of how to add custom middleware https://github.com/dai-shi/waku/blob/main/examples/08_cookies/start.js is nice, but isn't supported via waku's cli.
• First class support for an api.ts file allows for waku to parameterize useful things at runtime (in future PRs) that would otherwise be difficult in the 08_cookies example linked above. Ex: optional middleware, computed configuration, build info, etc. See defineApi here https://github.com/campbellman/waku/compare/main...campbellman:waku:issue-329-custom-middleware?expand=1#diff-f0671b5207dd962b56acf7e73f1dee3deda53ea547041db5484e5c6eb2ebf58eR54
• Allows for flexible deployments not defined in https://github.com/dai-shi/waku/tree/main/packages/waku/src/lib/builder. Developers can more easily target obscure platforms this way.
• Could provide a pathway towards not needing any of the serve-* or output-* files in https://github.com/dai-shi/waku/tree/main/packages/waku/src/lib/builder. These could exclusively live in the create-waku package as templates, and external templates could be supported as well.

Caveats

• Not sure what goes into making this work with deno (see current serve-deno.ts file: https://github.com/campbellman/waku/blob/main/packages/waku/src/lib/builder/serve-deno.ts)
• My implementation tightly couples the api.ts file to hono for the sake of simplicity. Would need more work to allow users to bring their own providers, eg: express, fastify, etc
• I'm new to vite, so my approach might need some rethinking. I basically piggy-backed on the current buildServerBundle function to add the api.ts file as an entry. But this might need its own config.

[campbellman]

This needs more thought, but the idea is to define platform specific handler logic here.