fix: constrain feature platforms in schema

[bollwyvl]

At present, the following does not raise an error when editing/validating:

[feature.foo_bar]
platforms = ["garbage"]

But does raise an issue when parsed:

  × failed to parse project manifest
    ╭─[pixi.toml:10:13]
  9 │ [feature.foo_bar]
 10 │ platforms = ["garbage"]
    ·             ─────┬─────
    ·                  ╰── 'garbage' is not a known platform. Valid platforms are 'noarch', 'unknown', 'linux-32', 'linux-64', 'linux-aarch64', 'linux-armv6l', 'linux-armv7l', 'linux-ppc64le', 'linux-ppc64', 'linux-s390x', 'linux-riscv32', 'linux-riscv64', 'osx-64', 'osx-arm64', 'win-32', 'win-64', 'win-arm64', 'emscripten-wasm32', 'wasi-wasm32', 'zos-z'
    ╰────

This PR:

• makes an enum out of Platform
  • updates it to the current list reported by the error
  • sorts the list
• uses Platform in platforms and feature.*.platforms.
• adds a schema test for bad feature platforms
  • reworks schema example finder to glob over (in)valid/*.toml

Future work (not on this PR):

• the "true" list of platforms is, of course, whatever the rust code (probably rattler?) thinks it is, but apparently, short of creating an error, there is no way to get this list
  • this could perhaps be added to
    • pixi info --json (which, as with More task JSON #1344, doesn't have a schema) or
    • something like $PIXI_ENVIRONMENT_PLATFORMS (not sure how useful that is outside of this case)

[ruben-arts]

Thank you! Would be best if this was validated to be in sync but I don't know a straight forward way to so now.