Add methods `enumerate` and `list.enumerate` that work like python `enumerate` about polars HOT 3 OPEN

df.with_columns(
   pl.col("val").list.eval(
      pl.struct(
         index = pl.cum_count(),
         value = pl.element()
      )
   )
)

# shape: (2, 2)
# ┌─────┬─────────────────────────────┐
# │ num ┆ val                         │
# │ --- ┆ ---                         │
# │ i64 ┆ list[struct[2]]             │
# ╞═════╪═════════════════════════════╡
# │ 1   ┆ [{1,"a"}, {2,"c"}, {3,"e"}] │
# │ 2   ┆ [{1,"b"}, {2,"d"}, {3,"f"}] │
# └─────┴─────────────────────────────┘

def enumerate(self, name: str = "index") -> pl.Expr:
    """Args: name (str, optional): Name of the index column. Defaults to "index"."""
    return pl.struct(
        pl.int_range(pl.count()).alias(name),
        self,
    ).alias(self.meta.output_name())


def list_enumerate(self, name: str = "index") -> pl.Expr:
    return pl.struct(
        pl.int_ranges(0, self.list.len()).alias(name),
        self,
    ).alias(self.meta.output_name())


pl.Expr.enumerate = enumerate
# Can't figure out how to monkey patch this onto the list namespace, but not the point
pl.Expr.list_enumerate = list_enumerate


df = pl.DataFrame({"num": [1, 2], "val": [["a", "c", "e"], ["b", "d", "f"]]})

print(
    df.select(
        "num",
        pl.col("val").enumerate().alias("plain_enumerate"),
        pl.col("val").list_enumerate().alias("list_enumerate"),
        pl.col("val").list.eval(pl.element().enumerate()).alias("eval_enumerate"),
    )
    # then to get the data into a completely flat format, do one of these
    # .unnest("list_enumerate").explode("index", "val")
    # the "val" col name is lost in the "eval_enumerate" because of `pl.element()` - will open an issue
    # .explode("eval_enumerate").unnest("eval_enumerate")
)

# shape: (2, 4)
# ┌─────┬─────────────────────┬─────────────────────────────┬─────────────────────────────┐
# │ num ┆ plain_enumerate     ┆ list_enumerate              ┆ eval_enumerate              │
# │ --- ┆ ---                 ┆ ---                         ┆ ---                         │
# │ i64 ┆ struct[2]           ┆ struct[2]                   ┆ list[struct[2]]             │
# ╞═════╪═════════════════════╪═════════════════════════════╪═════════════════════════════╡
# │ 1   ┆ {0,["a", "c", "e"]} ┆ {[0, 1, 2],["a", "c", "e"]} ┆ [{0,"a"}, {1,"c"}, {2,"e"}] │
# │ 2   ┆ {1,["b", "d", "f"]} ┆ {[0, 1, 2],["b", "d", "f"]} ┆ [{0,"b"}, {1,"d"}, {2,"f"}] │
# └─────┴─────────────────────┴─────────────────────────────┴─────────────────────────────┘