PR #5008 Better math argument parsing

Several adjustments have been made to argument parsing for function calls in math mode, including:

• named parameters now support any identifier (including hyphens - and underscores _)
• The .. spread operator can now be used in argument lists
• Passing the same named parameter multiple times now results in an error, rather than silently using the latter value

1
#let func(my-body: none) = my-body
2
$ func(my-body: a) $

1
$ mat(..#range(4).chunks(2)) $

1
  #let func(my: none) = my
2
$ func(my: a, my: b) $