Add fix for commas appearing within keyword arg str

[timlod]

It's possible for string keyword arguments to contain defaults that have commas, for example:

def fun(a: str = ",")
    pass

Currently, this would create an extra argument because of the comma within the string.
This commit adds a bypass for commas single- or double-quoted strings within keyword arguments much the same way that it ignores commas within the square brackets of type hints.

Example input + call:

(let ((s "(a: str = ',', b: int = 1)"))
  (sphinx-doc-split-args s))

Output previous:

("(a: str = '" "'" " b: int = 1)")

Output new:

("(a: str = ','" " b: int = 1)")