Append to punctuation buffer

[moewew]

Context https://github.com/plk/biblatex-apa/issues/126#issuecomment-687829069.

I think it may be useful to be able to append to the current unit punctuation (saved in \blx@unitpunct) instead of only being able to overwrite it with \setunit or \printunit. The new command could be called \appendunit or the like and at its heart would be

\long\def\blx@appendunit#1{%
  \iftoggle{blx@keepunit}
    {}
    {\gappto\blx@unitpunct{#1}%
     \global\toggletrue{blx@unit}}}

We'd need to create a starred version, double check how it should work with blx@keepunit/\printunit and think through some other implications of appending to the buffer.