Assists which always generate a snippet regardless of snippet capability

These assists always generate a snippet regardless of if the LSP client does or doesn't support the snippet text edit capability:

• extract_expressions_from_format_string https://github.com/rust-lang/rust-analyzer/pull/17333
• generate_constant
• generate_mut_trait_impl https://github.com/rust-lang/rust-analyzer/pull/20336

They unfortunately weren't captured in the initial batch of migrations to the new structured snippet API as they didn't use {insert,replace}_snippet, which is what I used to figure out which assists needed migrating.

Note that because the structured snippet API (add_{tabstop,placeholder}_*) relies on syntax nodes, they must also be migrated away from using insert and replace.

Start with crates/ide-assists/src/handlers/generate_constant.rs and compare it with assists already migrated to the structured snippet API. Check how snippet capability is handled, and identify the existing insert/replace usage that must be migrated. Done means generate_constant respects the LSP client's snippet support without relying on the old insertion methods.