Add a way to get psalm level data

[ricardoboss]

Hi! Psalm is a great project and the shepherd page makes it super easy to visualize type coverage and psalm level with badges. We have a way to get the raw coverage data and display them using the shields.io badges, but there is no way to do the same with the psalm level.

This PR adds this endpoint.

Example level data URL: https://shepherd.dev/github/elephox-dev/framework/level Example response:

{
    "schemaVersion": 1,
    "label": "psalm-level",
    "message": "1",
    "color":"#4c1"
}

[orklah]

Thanks for this :)

@weirdan I can't approve worflow here yet. Can you give me permissions?

[muglug]

@orklah I've added you!

[orklah]

Thanks Matt!

[orklah]

Thanks @ricardoboss !