Add definition for mensural-f3 clef

[Kim Shrier]

Hello,

I’m not sure of the current way to suggest a change to lilypond source
but I just have a simple, 1-line change to propose.

It would be useful to have a mensural F clef that is on staff line 3
for a baritone clef.

$ git diff
diff --git a/scm/parser-clef.scm b/scm/parser-clef.scm
index 9b692913a6..a2558a2055 100644
--- a/scm/parser-clef.scm
+++ b/scm/parser-clef.scm
@@ -75,6 +75,7 @@
     ("blackmensural-c3" . ("clefs.blackmensural.c" 0 0))
     ("blackmensural-c4" . ("clefs.blackmensural.c" 2 0))
     ("blackmensural-c5" . ("clefs.blackmensural.c" 4 0))
+    ("mensural-f3" . ("clefs.mensural.f" 0 0))
     ("mensural-f" . ("clefs.mensural.f" 2 0))
     ("mensural-g" . ("clefs.mensural.g" -2 0))
     ("neomensural-c1" . ("clefs.neomensural.c" -4 0))

Kim