feat(expr-ir): Add BaseFrame.unpivot

[dangotbanned]

Description

Next one in this series of gap-fillers is DslFunction::Unpivot

The only difference from main is what is allowed in on and index:

narwhals/narwhals/dataframe.py

Lines 2272 to 2276 in 83f4f2e

	def unpivot(
	self,
	on: str | list[str] | None = None,
	*,
	index: str | list[str] | None = None,

narwhals/narwhals/_plan/dataframe.py

Lines 162 to 166 in 83cb80e

	def unpivot(
	self,
	on: OneOrIterable[ColumnNameOrSelector] | None = None,
	*,
	index: OneOrIterable[ColumnNameOrSelector] | None = None,

Related issues

• Child of feat(RFC): A richer Expr IR #2572
• Follows feat(expr-ir): Impl DataFrame.unique #3364
• Towards oh-nodes...expr-ir/logical-plan