Recognize Rust function modifier combinations in the spacing helper #890

Problem

is_function_start uses a short prefix list. Valid pub(crate) const fn functions and other modifier combinations are not detected, leaving adjacent functions unseparated.

Reproduction

Executed the helper at fd07427f4a976de05e38eff3ee1760db97fbb761 using importlib and add_function_spacing on:

pub(crate) const fn first() {}
pub(crate) const fn second() {}

The result is unchanged; the intended separating blank line is missing. Starting point: tools/format-rust.py.

Scope

Recognize legal visibility and const/async/unsafe/extern combinations without treating arbitrary extern blocks or fn text in strings as functions. Difficulty: easy.

Completion criteria

• Cover combined modifiers and restricted visibility.
• Keep bodyless declarations and extern blocks distinct.
• Add a small table-driven Python regression.