build: replace luacheck with selene

Problem: luacheck is unmaintained (last release 2018) and required
suppressing four warning classes to avoid false positives. It also
lacks first-class vim/neovim awareness.

Solution: switch to selene with std='vim' for vim-aware linting.
Replace the luacheck CI job with selene, update the Makefile lint
target, and delete .luacheckrc.

.github/workflows/tests.yml

@@ -10,21 +10,14 @@ on:
       - main
 
 jobs:
-  luacheck:
-    name: Luacheck
+  selene:
+    name: Selene
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
-
-      - name: Prepare
-        run: |
-          sudo apt-get update
-          sudo add-apt-repository universe
-          sudo apt install luarocks -y
-          sudo luarocks install luacheck
-
-      - name: Run Luacheck
-        run: luacheck lua tests
+      - uses: NTBBloodbath/selene-action@v1.0.0
+        with:
+          args: --display-style quiet .
 
   stylua:
     name: StyLua