feat: simple platform specific dependencies

[baszalmstra]

Enables writing platform specific dependencies using:

[target.<platform>.dependencies]
foo = "3"

e.g.

[target.win-64.dependencies]
windows_specific = "3"

[target.osx-64.dependencies]
mac_specific = "3"

For now the key is just a platform. In the future we want to extend this to allow more generic expressions.

Fix #43

[ruben-arts]

Small improvements. Also, is there a good example we could test this with? I'm now just faking it.

[baszalmstra]

Ok I elaborated a bit on error reporting:

error: failed to parse pixi.toml from E:\Developer\prefix\pixi\myproject

Caused by:
    TOML parse error at line 3, column 11
      |
    3 | version = 3
      |           ^
    invalid type: integer `3`, expected a string

is now:

Error: failed to parse project manifest
   ╭─[pixi.toml:3:11]
   │
 3 │ version = 3
   │           ┬
   │           ╰── invalid type: integer `3`, expected a string
───╯

and when using a platform that is not added to the platforms array you get:

Error: Targeting a platform that this project does not support
    ╭─[pixi.toml:17:9]
    │
 17 │ [target.osx-64.dependencies]
    │         ───┬──
    │            ╰──── 'osx-64' is not a supported platform
    │
    │ Help: Add 'osx-64' to the `project.platforms` array of the pixi.toml manifest.
────╯