feat: implement form actions handlers

[Hebilicious]

🔗 Linked issue

nuxt/nuxt#20649
nuxt/nuxt#20852

https://github.com/Hebilicious/form-actions-nuxt

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR adds an experimental features that allows Nitro to supports fallback routes.

This is useful to register a post handler alongside the nuxt renderer.
nitro.config.handlers.push({ route: "/hello-world", handler: "~/actions/hello-world.ts", formAction: true })

Nitro will send an error if there's a form action handler but no renderer has been specified.

When hitting /hello-world with a post request, the registered handler will be used.
But when hitting /hello-world with a get request, the renderer will be used.

Something to potentially do in the future would be to add the action directory to the nitro scanners.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #1286 (96d261f) into main (1950d1e) will increase coverage by 0.09%.
The diff coverage is n/a.

❗ Current head 96d261f differs from pull request most recent head dabd367. Consider uploading reports for the commit dabd367 to get more accurate results

@@            Coverage Diff             @@
##             main    #1286      +/-   ##
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+ Coverage   76.73%   76.83%   +0.09%     
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  Files          71       68       -3     
  Lines        7273     6829     -444     
  Branches      727      692      -35     
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- Hits         5581     5247     -334     
+ Misses       1691     1580     -111     
- Partials        1        2       +1     

see 24 files with indirect coverage changes

[Hebilicious]

@pi0 This is ready for review, I followed the approach that you suggested with a separated router for form actions

[pi0]

With the upstream fix in h3 (unjs/h3#461), we can actually register form action routes with their method and no additional configuration and they are expected to work nicely with the default renderer catch-all route.

[Hebilicious]

Thanks @pi0, I can confirm that everything is working smoothly.
I've upgraded https://github.com/Hebilicious/form-actions-nuxt/releases/tag/v0.2.0-beta.0 to use the latest nitro and h3 🎉