Before the delta input config is stored, this attempts to compile a
keymap with it. If the keymap fails to compile, then the first line of
the xkbcommon log entry will be included with a `CMD_FAILURE`, the
entire xkbcommon log entry will be included in the sway error log, and
the delta will not be stored.
This only handles basic issues such as a layouts not existing. This
will NOT catch more complex issues such as when a variant does
exist, but not for the given layout (ex: `azerty` is a valid variant,
but the `us` layout does not have a `azerty` variant).
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