Don't mask implicit mods

[urob]

Small PR to the PR: This prevents masked-mods from masking implicit modifiers.

This enables configs that bind a mod-morph to things like LS(LC(U)). The proposed change will send implicitly specified modifiers even when included in the masked_mods argument. There are two advantages:

1. The current behavior which masks out implicit modifiers that are explicitly specified as part of the binding is unintuitive. See the review comments by @slashfoo on the PR
2. It provides a way to bind macros that (at some point) make use of a masked-mod. This is essential, for example, for implementing shifted/unshifted unicode pairs via ZMK macros (see here for an example)

EDIT: I added a few more additional tweaks to the PR:

• I deleted an used definition of trigger_mods
• After discussing it on discord, I set masked_mods to mods if unspecified by the user. This resolves the issues raised in when using mod-morph to make shift-backspace work as delete, the shift key is not 'swallowed' zmkfirmware/zmk#683
• I updated the docs with the new options

EDIT: Closed after discussing with Pete on discord and deciding that it might be better to open a new PR with the changes. Link to the new PR: zmkfirmware#1412