Make PREVENT_STUCK_MODIFIERS the default (#3107)

* Remove chording as it is not documented, not used, and needs work.

* Make Leader Key an optional feature.

* Switch from `PREVENT_STUCK_MODIFIERS` to `STRICT_LAYER_RELEASE`

* Remove `#define PREVENT_STUCK_MODIFIERS` from keymaps.

docs/feature_leader_key.md

@@ -39,3 +39,11 @@ void matrix_scan_user(void) {
 As you can see, you have a few function. You can use `SEQ_ONE_KEY` for single-key sequences (Leader followed by just one key), and `SEQ_TWO_KEYS`, `SEQ_THREE_KEYS` up to `SEQ_FIVE_KEYS` for longer sequences.
 
 Each of these accepts one or more keycodes as arguments. This is an important point: You can use keycodes from **any layer on your keyboard**. That layer would need to be active for the leader macro to fire, obviously.
+
+## Adding Leader Key Support in the `rules.mk`
+
+To add support for Leader Key you simply need to add a single line to your keymap's `rules.mk`:
+
+```
+LEADER_ENABLE = yes
+```