feat: add fmt_flag() method

[rich-iannone]

This adds the fmt_flag() method, which allows for the rendering of flag icons based on 2- or 3-letter country codes in table cells. The flag SVG data has been added within CSV data so there are no new dependencies with this PR.

Here's an example of the fmt_flag() method in use:

from great_tables import GT
from great_tables.data import countrypops
import polars as pl

countrypops_mini = (
    pl.from_pandas(countrypops)
    .filter(pl.col("year") == 2021)
    .filter(pl.col("country_name").str.starts_with("S"))
    .sort("country_name")
    .head(10)
    .drop(["year", "country_code_3"])
)

(
    GT(countrypops_mini)
    .fmt_integer(columns="population")
    .fmt_flag(columns="country_code_2")
    .cols_label(
        country_code_2 = "",
        country_name = "Country",
        population = "Population (2021)"
    )
    .cols_move_to_start(columns="country_code_2")
)

[codecov]

Codecov Report

Attention: Patch coverage is 87.71930% with 7 lines in your changes missing coverage. Please review.

Project coverage is 89.78%. Comparing base (925fa41) to head (ffd71b3).

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #523      +/-   ##
==========================================
- Coverage   89.79%   89.78%   -0.01%     
==========================================
  Files          45       45              
  Lines        5321     5376      +55     
==========================================
+ Hits         4778     4827      +49     
- Misses        543      549       +6     

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[machow]

This file is 1.74 MB, which is a bit large to carry around in a Python library. Is the purpose of this file so people can specify flag names in a variety of locales? I wonder if 1.74MB in order to support specification of flag names across locales is a bit heavy (could there be a separate library for translating country names in different locales?).

[rich-iannone]

Yeah, I think a separate library might be better for this extra feature. I'll slim this down.