refactor: rename SupportSchema -> SchemaLike, fix type definition

[jcrist]

This is a follow-up to #8421.

• The use of a TypeVar here was incorrect - what you want is a TypeAlias instead. The previous version would cause type checkers to error.
• I moved the type definition to be adjacent to Schema. This makes the import paths cleaner since you don't need as many if TYPE_CHECKING blocks. Logically it also makes sense to me to put the union of schema-like things next to the Schema code.
• I renamed the type alias to SchemaLike which reads better IMO.

[cpcloud]

Thanks!