Implementation of validate for Model Statement and Scalar.

[marron-at-work]

Adds support for validate on Model Statements and Scalars (issue #2041):

 model M {
    ii: int64;

    @doc("Check that ii is greater than or equal to 0")
    validate chkii: ii >= 0;
}

scalar S extends int64 {
    validate chkv: value >= 0;
}

Tests for basic functionality.
TODO: prettier formatting.

[github-actions]

Changes in this PR will be published to the following url to try(check status of TypeSpec Pull Request Try It pipeline for publish status):
Playground: https://cadlplayground.z22.web.core.windows.net/prs/2100/

Website: https://cadlwebsite.z1.web.core.windows.net/prs/2100/

[bterlson]

I think we should make the id optional. It is super annoying to have to add an id, but will be needed depending on how you want to use it. Linter can of course enforce that you have id.

[bterlson]

I feel like validates reads better but I don't feel super strongly. Anyone else have feelings?

[bterlson]

I've taken a look at the parser and related bits, looks great, you handled a lot of the fiddly bits. I think the only thing missing is an update to the grammar file and keyword colorization - see src/server/tmlanguage.ts. May also need updates to getSemanticTokens since validate is a conditional keyword, though would have to check what we're doing for other conditional keywords.

I will check into the checker bits tomorrow!

[bterlson]

@marron-at-work I think this is superceded by #2141?