Add a mix.yaml preset that enables mix_lint

[leoafarias]

Summary

• Add package:bitwild_analysis/mix.yaml, which enables Mix's mix_lint analyzer plugin. Projects that use Mix include it next to plugin.yaml (or plugin_all.yaml) and get both plugins from one include: list.
• Each preset declares only its own plugin: the analyzer merges plugins: blocks from separate included files.
• mix_lint comes from the Mix repository's main branch, because the pub.dev release (2.0.0) can't load alongside bitwild_lints (older analyzer range, no lib/main.dart). Switch to a pub.dev version once a fixed release ships. Style rule names aren't listed, since refactor(mix_lint)!: rename rules, fix false positives, and align with Mix style mix#1083 renames them; mix_lint's warnings are on by default.
• README, install guide and changelog updated.

Validation

• Throwaway Flutter project on Dart 3.13.3 (Flutter 3.47.3), depending on Mix main by path and on this branch's bitwild_analysis from Git, including flutter.yaml, plugin.yaml and mix.yaml: dart analyze reported unconditional_recursion (bitwild_lints) and mix_avoid_empty_variants (mix_lint) on deliberate violations.
• Same check with the two plugins declared in two separately included local files: both reported.