Feat: Allow spanners to use units notation in .tab_spanner()

[rich-iannone]

This PR allows for the use of units notation text in spanner labels (through .tab_spanner()). Here is an example of the intended usage:

from great_tables import GT, md, system_fonts
from great_tables.data import reactions
import polars as pl
import polars.selectors as ps

reactions_mini = (
    pl.from_pandas(reactions)
    .filter(pl.col("cmpd_type") == "mercaptan")
    .select([
        "cmpd_name",
        "cmpd_formula",
        ps.ends_with("k298")
    ])
    .with_columns(
        cmpd_formula=pl.concat_str(
            "%" + pl.col("cmpd_formula") + "%"
        )
    )
)

(
    GT(reactions_mini, rowname_col="cmpd_name")
    .tab_header(title="Gas-phase reactions \
    of selected mercaptan compounds")
    .tab_spanner(
        columns=ps.ends_with("k298"),
        label = "Reaction Rate Constant (298 K),<br>{{cm^3 molecules^–1 s^–1}}"
    )
    .fmt_units(columns="cmpd_formula")
    .fmt_scientific(columns=ps.ends_with("k298"))
    .sub_missing()
    .cols_hide(columns="O3_k298")
    .cols_label(
        cmpd_formula="",
        OH_k298="OH",
        NO3_k298="{{%NO3%}}",
        Cl_k298="Cl",
    )
    .opt_stylize(style=1, color="blue")
    .opt_horizontal_padding(scale=3)
    .tab_options(
        table_font_names=system_fonts("humanist"),
    )
)

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.43%. Comparing base (793bd2a) to head (aa219aa).

Files	Patch %	Lines
great_tables/_spanners.py	70.00%	3 Missing ⚠️
great_tables/_boxhead.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
- Coverage   86.48%   86.43%   -0.06%     
==========================================
  Files          42       42              
  Lines        4685     4696      +11     
==========================================
+ Hits         4052     4059       +7     
- Misses        633      637       +4     

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[machow]

LGTM! I'll open an issue to document this behavior in tab_spanner, but want to get this tucked away!

[machow]

#395