This makes the "wordchars" setting from filetypes.common and each
specific filetype override filetype.common's "whitespace_chars"
setting, rather than it overriding filetype-specific "wordchars".

This makes the it easy to chose filetype-specific "wordchars", where
before user had not only to update this setting, but also the
filetype.common "whitespace_chars" setting if it listed one or more of
the new characters for the change to actually have an effect -- and
changing "whitespace_chars" for every filetype.

Closes #3429368.