feat(expr-ir): Add read_{csv,parquet}

[dangotbanned]

Description

Act 4 in this adventure is towards DslPlan::Scan.

Like (#3369), this will be a very trimmed down version to cover scan_csv and scan_parquet

narwhals/narwhals/_plan/logical_plan/plan.py

Lines 98 to 107 in 63dbc65

	# TODO @dangotbanned: Add `ScanCsv`, `ScanParquet` (after adding them in #2572)
	# TODO @dangotbanned: Maybe rename this to `Scan` for consistency with polars
	class Source(LogicalPlan):
	"""Root node of a `LogicalPlan`."""
	def iter_left(self) -> Iterator[LogicalPlan]:
	yield self
	def iter_right(self) -> Iterator[LogicalPlan]:
	yield self

Related issues

• Child of feat(RFC): A richer Expr IR #2572
• Follows feat(expr-ir): Add DataFrame.write_{csv,parquet} #3369
• Towards oh-nodes...expr-ir/logical-plan