r-a: Opt-in for the proc-macro expansion feature

Bilal Elmoussaoui requested to merge bilelmoussaoui/rust-analyzer into main

The proc-macro feature in r-a is still experimental, although without it the experience with code that uses proc macros is sub-optimal.

As gtk-rs apps (the most common use case for Builder/Rust) make heavy use of macros let us opt-in for that by default

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