Add extension placeholder

[puneetdixit200]

Summary:

• add a {.ext} formatter token that expands to the current path's file extension
• wire it through --format, --exec, and --exec-batch placeholder parsing
• add unit and integration coverage, CLI help text, and a changelog entry

I used a single {.ext} placeholder because Rust's Path::extension() is based on the final path component, so {.ext} and {/.ext} would return the same value.

AI assistance: an AI coding assistant helped draft the patch and tests. I reviewed the change and ran the verification below.

Refs #1818

Tests:

• CARGO_HOME=/tmp/fd-1818-cargo-home CARGO_TARGET_DIR=/tmp/fd-1818-target cargo test fmt::fmt_tests::extension_placeholder -- --nocapture
• CARGO_HOME=/tmp/fd-1818-cargo-home CARGO_TARGET_DIR=/tmp/fd-1818-target cargo test exec::tests::tokens_with_extension -- --nocapture
• CARGO_HOME=/tmp/fd-1818-cargo-home CARGO_TARGET_DIR=/tmp/fd-1818-target cargo test fmt::input::path_tests::extension_simple -- --nocapture
• CARGO_HOME=/tmp/fd-1818-cargo-home CARGO_TARGET_DIR=/tmp/fd-1818-target cargo test --test tests format -- --nocapture
• CARGO_HOME=/tmp/fd-1818-cargo-home CARGO_TARGET_DIR=/tmp/fd-1818-target cargo test --test tests test_exec -- --nocapture
• CARGO_HOME=/tmp/fd-1818-cargo-home CARGO_TARGET_DIR=/tmp/fd-1818-target cargo fmt -- --check
• CARGO_HOME=/tmp/fd-1818-cargo-home CARGO_TARGET_DIR=/tmp/fd-1818-target cargo test
• CARGO_HOME=/tmp/fd-1818-cargo-home CARGO_TARGET_DIR=/tmp/fd-1818-target cargo clippy --all-targets -- -D warnings
• git diff --check