Add named schemas for missing built in types

[jamesrswift]

Added the following missing common types:

• Gradient
• Schema
• Version
• bytes
• function
• location
• plugin
• regex
• selector
• label
• alignment
• angle
• direction
• fraction
• length
• ratio
• relative
• symbol

Prior to merge, ensure that:

• Docs are up to date
• All newly added types have sufficient coverage in tests

[tingerrr]

Makes sense overall, I've attached a list of types that I compiled when I scraped the Typst docs manually.

[tingerrr]

Looks good, I think we can add most of them as they are.

But I think we should (in another PR) add some coercions for some of those types, like stroke.

Some of them may also need default post-transformations.

[tingerrr]

I'm a little concerned about this one, length and ratio will not have the same methods as relative for example, z.parse would return whatever type is passed without converting it to a relative.

This can be problematic, as shown here.

Should we be less permissive about the types or add a default post-transformation?