[Feature] Send a dummy keycode to neutralize flashing modifiers in retro tap and key overrides

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Description

Certain QMK features like Retro Tap and Key Overrides must unregister a previously registered modifier before sending a certain keycode but in doing so, the OS can sometimes misinterpret the intent of the user and trigger a keyboard shortcut when seeing the tap of modifier key. Notable examples are taps of the GUI key opening the start/applications menu in many desktop environments like Windows or Gnome or taps of the left Alt key focusing the menu bar in many GUI applications. This causes problems for the users of these features as described in issue #16795.

This PR aims to solve this issue by the neutralization of those flashing modifiers achieved through the tap of a dummy, user-configurable, keycode.

To illustrate; the idea is to send a keycode like Right Control so that the OS sees the following key events when retro-tapping LGUI_T(KC_A):

Left GUI down
Right Control down
Right Control up
Left GUI up
A down
A up

LGUI + RCtrl is an extremely rare keyboard shortcut which is unlikely to be bound to any action, so doing so amounts to a no-op and we can thus simply send "a" with no other side-effects, just as the user intended.

All the user has to do is to define two options in config.h:

#define DUMMY_MOD_NEUTRALIZER_KEYCODE <kc>
#define MODS_TO_NEUTRALIZE { <mod_mask_1>, <mod_mask_2> }

with sensible defaults for MODS_TO_NEUTRALIZE already available.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• [Bug]Keys override: Nearly unusable with modifiers like alt or LGUI on windows, or in an app with a menu #16795
• [Bug] RETRO_TAPPING does not work with ALT_T(*) #9230

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[precondition]

I am open to other naming suggestions