refactor: Add more checks to pre-commit

Updated existing pre-commit hooks and added some new hooks:

- Remove trailing whitespace
- Ensure every non-empty file ends with a new line
- Check YAML file validity
- Prevent adding large files
- Ensure any scripts with shebangs are executable

Added a GitHub action to run pre-commit on every commit. Removed any
existing actions which duplicate pre-commit.

Ran pre-commit on the codebase.

app/boards/shields/two_percent_milk/two_percent_milk.keymap

@@ -7,11 +7,11 @@
 #include <behaviors.dtsi>
 #include <dt-bindings/zmk/keys.h>
 #include <dt-bindings/zmk/bt.h>
- 
+
 / {
     keymap {
         compatible = "zmk,keymap";
- 
+
         default_layer {
             bindings = <
                 &kp X