chore: add deprecation notice for type ascription use

[saiintbrisson]

Adds a deprecation notice when type ascription pattern is used, preparing grounds for when
we drop support for the pattern, keeping compatibility with Rust's supported syntax, as decided
on RFC 3307.

This change was created as discussed in #2482.

Example

The following code uses type ascription for the second argument.

    let _ = sqlx::query!(
        r#"SELECT $1 as a, $2 as b"#,
        foo_bar as i32,
        (6 + 8): <Foo as Deref>::Target
    );

Generating a warning that looks as follows:

warning: use of deprecated constant `main::{closure#0}::warning_arg1`: 
                 Type ascription pattern is deprecated, prefer casting
                 Try changing from
                     `(6 + 8) : < Foo as Deref > :: Target`
                 to
                     `(6 + 8) as < Foo as Deref > :: Target`
         
                 See <https://github.com/rust-lang/rfcs/pull/3307> for more information
  --> src/main.rs:16:9
   |
16 |         (6 + 8): <Foo as Deref>::Target
   |         ^^^^^^^

[saiintbrisson]

The formatting is a bit clunky with all the spaces, but this is how syn prints it and there isn't much we can do. Though I don't see this as a crucial problem.

EDIT: I also couldn't get the span to wrap the entirety of the expression for some reason. It completely ignores the type override part.

[saiintbrisson]

The names won't conflict as the consts are created inside the invocation's local scope.

[saiintbrisson]

@abonander should I enable the missing syn features for sqlx-macros on this PR or will there be a separate PR for this? Referring to the 0.6.3 changelog.

[abonander]

We enable the correct features in sqlx-macros-core so enabling them in sqlx-macros isn't strictly necessary. I would only enable the features that sqlx-macros specifically uses.

[saiintbrisson]

Done, I've enabled both "parsing" and "proc-macro", which were missing and causing my local test to fail while building without syn on the build-dependencies.