fix: properly capture lvalues in closure environments (#2120)

[alexvitkov]

Description

Problem*

Work towards resolving #2120

Summary*

Previously LValues weren't scanned & added to closure environments. This PR fixes that.

However, the example given in #2120 still doesn't work due to an unrelated codegen issue (#2255):

fn main() {
    let x1 = &mut 42;
    let set_x1 = |y| { *x1 = y; };

    assert(*x1 == 42);
    set_x1(44);
    assert(*x1 == 44);
    set_x1(*x1);
    assert(*x1 == 44);
}

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.