[ty] Track the source ranges of reStructuredText field lists in docstrings (#25751)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Building towards https://github.com/astral-sh/ty/issues/3454, this
augments the parsed representation of a reStructuredText field list to
include the source range of that list in the original docstring. This
has no external effect for the moment, but will later enable us to
precisely replace regions of the original docstring with Markdown.
## Test Plan
Please see included tests.
<!-- How was it tested? -->
ruff
49f253d7 - [ty] Track the source ranges of reStructuredText field lists in docstrings (#25751)
