Improve speed of quantity stringification

[mattwthompson]

Relates to #2065, split off from #2081

I see a ~20% speedup in stringifying some force fields

In [1]: import openff.toolkit

In [2]: ff14sb = openff.toolkit.ForceField("ff14sb_off_impropers_0.0.4.offxml")
/Users/mattthompson/micromamba/envs/openff-toolkit-test/lib/python3.12/site-packages/openff/amber_ff_ports/amber_ff_ports.py:8: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
  from pkg_resources import resource_filename
/Users/mattthompson/micromamba/envs/openff-toolkit-test/lib/python3.12/site-packages/openff/amber_ff_ports/amber_ff_ports.py:8: UserWarning: Module openff was already imported from None, but /Users/mattthompson/software/openff-toolkit is being added to sys.path
  from pkg_resources import resource_filename

In [3]: %timeit ff14sb._to_smirnoff_data()
388 ms ± 2.91 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

This is compared to

501 ms ± 24.4 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

upstream, on my machine

• Tag issue being addressed
• Add tests
• Update docstrings/documentation, if applicable
• Lint codebase
• Update changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.61%. Comparing base (158117d) to head (a3a6010).
Report is 1 commits behind head on main.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[j-wags]

Glorious, contender for best PR of the year. Please update releasenotes before merging to claim some credit :-)