Fix doubled comma when inserting into a comma-first array

[gaoflow]

Fixes #494.

In a comma-first formatted array the comma that separates an item from the previous one lives in that item's indent whitespace (the parser folds \n , into the next group's indent when a comment ends the previous group). Array.insert() copies such an indent onto the new item, but then also adds a trailing comma to the previous item — and for middle inserts gives the new item a trailing comma too — so the output contains double separators and no longer parses:

doc = tomlkit.parse("a = [\n      1 # one\n     ,2\n    ]\n")
doc["a"].append(99)
tomlkit.parse(tomlkit.dumps(doc))  # UnexpectedCharError: ',' — dumped "2,\n     ,99"

The fix skips both extra commas when the indent copied onto the new item already carries one: the embedded comma is the separator. Output is style-preserving (,2 → ,99) and round-trips. Same structure as the __delitem__ comma_in_indent handling from #486, which insert() never got.

Regression tests cover append, middle insert (both previously produced unparseable output), and front insert (comma-less indent, unchanged behavior).

[frostming]

Thanks