feat: add fmt_icon() method

[rich-iannone]

This PR adds the fmt_icon() method. It depends on Font Awesome icons available in the faicons library, so that is a new dependency. Whenever that package is updated, new icons from the FA free set will be available. Here's an example of the fmt_icon() method in use:

import pandas as pd
from great_tables import GT
animals_foods_df = pd.DataFrame(
    {
        "animals": ["hippo", "fish,spider", "mosquito,locust,frog", "dog,cat", "kiwi-bird"],
        "foods": ["bowl-rice", "egg,pizza-slice", "burger,lemon,cheese", "carrot,hotdog", "bacon"],
    }
)
(
    GT(animals_foods_df)
    .fmt_icon(
        columns=["animals", "foods"],
        height="4em"
    )
    .cols_align(
        align="center",
        columns=["animals", "foods"]
    )
)

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 7 lines in your changes missing coverage. Please review.

Project coverage is 89.08%. Comparing base (aa7d553) to head (0e9e9e6).

Files with missing lines	Patch %	Lines
great_tables/_formats.py	85.41%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #515      +/-   ##
==========================================
- Coverage   89.11%   89.08%   -0.04%     
==========================================
  Files          44       44              
  Lines        5220     5269      +49     
==========================================
+ Hits         4652     4694      +42     
- Misses        568      575       +7     

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