Support clearing columns via DataTable.clear()

[JoshKarpel]

Looks like this feature must have gotten lost in the shuffle, since the docstring mentions it but the function doesn't support it:

textual/src/textual/widgets/_data_table.py

Lines 315 to 328 in 3aaa4d3

	def clear(self) -> None:
	"""Clear the table.
	Args:
	columns (bool, optional): Also clear the columns. Defaults to False.
	"""
	self.row_count = 0
	self._clear_caches()
	self._y_offsets.clear()
	self.data.clear()
	self.rows.clear()
	self._line_no = 0
	self._require_update_dimensions = True
	self.refresh()

Happy to do a PR for this. Is it as simple as running self.columns.clear() if the columns argument is True?

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