chore: enforce more rigorous code with clippy

[jeertmans]

What

Currently, only a few clippy rules are used, see:

languagetool-rust/src/lib/lib.rs

Lines 1 to 6 in 6cf960a

	#![deny(missing_docs)]
	#![deny(missing_debug_implementations)]
	#![warn(clippy::must_use_candidate)]
	#![allow(clippy::doc_markdown, clippy::module_name_repetitions)]
	#![cfg_attr(docsrs, feature(doc_auto_cfg))]
	#![doc = include_str!("../../README.md")]

I'd like to enforce a more uniform code style by adding other clippy rules, such as clippy::pendatic.

How to

It's a very open issue, and I would love to get suggestion from the community, on some clippy rules that I could use.
Of course, using some new rules might need to edit the current code base.