Specifying font-series for system font.

[Mike iLL Kilmer]

Hi all and happy new year.

To overwrite the typeface/font in LyricText I'm doing this:

```words = \lyricmode { 

 \override LyricText.font-name = "American Typewriter" 

I am a ly -- ric

}```

Where "American Typerwriter" is a TrueType font located at `/System/Library/Fonts/Supplemental/AmericanTypewriter.ttc` and comes up in the output of `#(ly:font-config-display-fonts)`:

family American Typewriter

 American Typewriter:style=Semibold

family American Typewriter

 American Typewriter:style=Bold

family Academy Engraved LET

 Academy Engraved LET:style=Plain

family American Typewriter

 American Typewriter:style=Condensed

Etc...

However I can't seem to get the `bold` or `semibold` series to render:

Have tried various cases and variants on `\override LyricText.font-series = #'bold`.

Seems it's looking for a symbol and not a string.

As always input gratefully appreciated.

–Mike