Deduplicate range/gen_series nested functions code

[Jefffrey]

Which issue does this PR close?

• Doing some prework for Internal error with generate_series: Internal error: could not cast array of type Date32 #15881

Rationale for this change

Range and GenSeries are essentially the same except for whether they include upper bounds or not; unify their function code to reduce duplication, making future changes easier.

What changes are included in this PR?

Remove GenSeries struct, folding it into Range. Do some more minor refactoring to their code.

Are these changes tested?

Existing tests (updated some error messages).

Are there any user-facing changes?

Not really (updated some error messages).

[Jefffrey]

Just like what window functions do:

datafusion/datafusion/functions-window/src/macros.rs

Lines 96 to 105 in 155b56e

	macro_rules! get_or_init_udwf {
	($UDWF:ident, $OUT_FN_NAME:ident, $DOC:expr) => {
	get_or_init_udwf!($UDWF, $OUT_FN_NAME, $DOC, $UDWF::default);
	};
	($UDWF:ident, $OUT_FN_NAME:ident, $DOC:expr, $CTOR:path) => {
	paste::paste! {
	#[doc = concat!(" Returns a [`WindowUDF`](datafusion_expr::WindowUDF) for [`", stringify!($OUT_FN_NAME), "`].")]
	#[doc = ""]
	#[doc = concat!(" ", $DOC)]

So we can use same Range struct with different constructor methods

[Jefffrey]

Custom constructor here

[Jefffrey]

Empty struct just so I can use the user_doc attribute; I was going to do it like this:

datafusion/datafusion/functions-window/src/rank.rs

Lines 107 to 140 in 155b56e

	static RANK_DOCUMENTATION: LazyLock<Documentation> = LazyLock::new(|| {
	Documentation::builder(
	DOC_SECTION_RANKING,
	"Returns the rank of the current row within its partition, allowing \
	gaps between ranks. This function provides a ranking similar to `row_number`, but \
	skips ranks for identical values.",
	"rank()")
	.with_sql_example(r#"
	```sql
	-- Example usage of the rank window function:
	SELECT department,
	salary,
	rank() OVER (PARTITION BY department ORDER BY salary DESC) AS rank
	FROM employees;
	+-------------+--------+------+
	| department | salary | rank |
	+-------------+--------+------+
	| Sales | 70000 | 1 |
	| Sales | 50000 | 2 |
	| Sales | 50000 | 2 |
	| Sales | 30000 | 4 |
	| Engineering | 90000 | 1 |
	| Engineering | 80000 | 2 |
	+-------------+--------+------+
	```
	"#)
	.build()
	});
	fn get_rank_doc() -> &'static Documentation {
	&RANK_DOCUMENTATION
	}

But I prefer using the user_doc attribute as it looks nicer imo; happy to switch if this isn't recommended

[Jefffrey]

Pulled these into the impl Range so they can easily access self.name() and self.include_upper_bound

[Jefffrey]

Looking to do this for #15881 itself