Implement the tab_style() method

[rich-iannone]

This adds the tab_style() method. The method requires locations and style values which are provided by new classes in the loc and style submodules.

You could do this:

import great_tables as gt
from great_tables import style, loc
from great_tables import exibble

(
    gt.GT(exibble)
    .tab_style(
        style=style.text(color="blue"),
        locations=loc.body(columns="num", rows=[0, 1, 2, 3]),
    )
    .tab_style(
        style=style.text(size="25px"),
        locations=loc.body(columns=["fctr"], rows=[0, 1, 2, 3]),
    )
    .tab_style(
        style=style.text(color="red"),
        locations=loc.body(columns=["fctr"], rows=[1, 2, 3, 4]),
    )
    .tab_style(
        style=style.text(size="10px"),
        locations=loc.body(columns=["fctr"], rows=[0, 1, 2, 3]),
    )
)

Fixes: #63