feat(python,rust,cli): add INITCAP string function for SQL

[alexander-beedie]

Adds support for the SQL equivalent of to_titlecase (to which this maps).

Example

import polars as pl
df = pl.DataFrame( { "words": ["Test SOME words"] } )

with pl.SQLContext( frame=df ) as ctx:
    res = ctx.execute(
        """
        SELECT
          words,
          INITCAP(words) as cap,
          UPPER(words) as upper,
          LOWER(words) as lower,
        FROM frame
        """
    ).collect()
    
# shape: (1, 4)
# ┌─────────────────┬─────────────────┬─────────────────┬─────────────────┐
# │ words           ┆ cap             ┆ upper           ┆ lower           │
# │ ---             ┆ ---             ┆ ---             ┆ ---             │
# │ str             ┆ str             ┆ str             ┆ str             │
# ╞═════════════════╪═════════════════╪═════════════════╪═════════════════╡
# │ Test SOME words ┆ Test Some Words ┆ TEST SOME WORDS ┆ test some words │
# └─────────────────┴─────────────────┴─────────────────┴─────────────────┘

[universalmind303]

since INITCAP is Oracle specific, I'm wondering if we just want to call it TITLECASE instead?

[alexander-beedie]

since INITCAP is Oracle specific, I'm wondering if we just want to call it TITLECASE instead?

No need - it's native PostgreSQL too (something I learned today, lol...) ;)
https://www.postgresql.org/docs/15/functions-string.html

[alexander-beedie]

Oops; left this for a while 😅
Updated/rebased against the latest code and all seems green.