feat: add style.from_column, implement for loc.body

[machow]

This PR implements the from_column() behavior from gt, which allows for setting a style based on values in the data.

e.g.

import pandas as pd
from great_tables import GT, exibble, loc, style

df = pd.DataFrame({"x": [1, 2], "color": ["red", "blue"]})

gt = GT(df)
gt.tab_style(
    style = style.text(color = style.from_column("color")),
    locations = loc.body(columns = ["x"])
)

NOTES:

• scope: currently only implemented for setting in loc.body(), raises if you try to use it with loc.title()
• implementation: the R gt library handles from_column inside each fmt_* function. This PR puts the handling inside set_style, by implementing a CellStyle._from_row() method. This gives each style the chance to return a new version of itself based on the data, just before setting it on the GT object.
• importing: I put it under style.from_column(), but another place it could go is with helpers like md() at the top-level of the package?

[machow]

Reference page!

[rich-iannone]

This is great! I think the main changes needed (as discussed) are: (1) having from_column() is not part of style, (2) enabling support for pl.col() from Polars.

[machow]

Alright, now we have:

• support for polars expressions in addition to from_colums()
• from_columns now only exported at top-level (i.e. from great_tables import from_column)

Note that I added a new internal class called FromValues, to represent a materialized column of values (e.g. from the polars Series that results from executing the polars expression). I didn't move FromColumn out of _styles.py, but that was mostly me being unsure of where to put it. I wonder if we might just leave it there until it gets used for other things? (e.g. fmt_*)

(down for anything though!)

[machow]

Here's the example from the from_column docs

[rich-iannone]

LGTM!!!

[machow]

noyce