Add support for var decl spans

[sophiajt]

Description

Remembers the var decl span in the variable's record. This helps us do better when getting the spans for an error make so we don't accidentally get the span of the closure.

fixes #4784 #4460

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass

[kubouch]

I added #4460 to the fixed list. Seems like the same issue.