Prefer language server formatting

[Boulea7]

Summary

• prefer formatting-capable language servers over configured external formatters
• use the external formatter when LSP formatting is unavailable or disabled with except-features = ["format"]
• document the precedence and cover both paths with integration tests

Closes #9543

Test plan

• cargo test -p helix-term --features integration --test integration test::commands::write::
• cargo test -p helix-view
• cargo clippy -p helix-view -p helix-term --features helix-term/integration --tests -- -D warnings
• cargo fmt --all -- --check
• cargo xtask docgen