Switched the GPIO matrix driver to debouncing using a simple integrator
algorithm. Whenever a key is pressed, we now scan at a rate controlled
by debounce-scan-period-ms (default 1 ms) until all keys are released,
then return to either waiting for an interrupt or polling more slowly.

The timers for key press and release can now be controlled separately,
so debounce-period is deprecated in favor of debounce-press-ms and
debounce-release-ms. Global Kconfig options
ZMK_KSCAN_DEBOUNCE_PRESS_MS and ZMK_KSCAN_DEBOUNCE_RELEASE_MS are also
added to make these easier to set.

Added documentation for debouncing options.

Refactored the GPIO matrix kscan driver so that only the data and config
structures are defined in the foreach macro. Functionality is unchanged
except for the addition of DT properties to adjust polling speed.

This should make it easier to add other enhancements later, like
improved and customizable debounce behavior.

PR: #400