fix(macros): only take an integer type from repr in the Type derive

[LckyLke]

Fixes #4366.

#[derive(sqlx::Type)] reads the enum's #[repr(...)] to find the integer type of a weak enum. It took the first plain identifier in the list, so #[repr(C)] produced MyEnum::Variant as C and bounds like C: Encode<'q, DB>, which do not compile.

The derive now only accepts an integer type from repr: i8 to i128, u8 to u128, isize, and usize. Markers such as C, transparent, and packed are ignored. An enum with only #[repr(C)] is therefore a strong enum, mapped by variant name, the same as an enum without any repr. For an enum without fields that is the only form repr(C) can take, because Rust rejects repr(C) combined with an integer repr on such enums.

A side effect is that a struct with a non-integer repr, for example #[repr(transparent)] on a #[sqlx(transparent)] newtype, no longer fails with "unexpected #[repr(..)]". That check only makes sense for an integer repr.

Added two cases to tests/sqlite/derives.rs: a #[repr(C)] enum round-trips as text, and a #[repr(transparent)] transparent struct round-trips as an integer. Ran with:

DATABASE_URL=sqlite://$PWD/tests/sqlite/sqlite.db cargo test --no-default-features --features sqlite,macros,runtime-tokio --test sqlite-derives