Add documentation hover for assignments with docstring

[oyarsa]

Summary

This PR adds attribute docstrings (PEP 257) to hover from assignments, including type aliases.

Fixes #2620

Test Plan

• test.py passes. It generated no changes to be commited. There are some compilation warnings, but I believe they are unrelated to the code in this PR.
• Tested as the LSP on neovim 0.11.6 with default lspconfig settings. I copied target/release/pyrefly to a directory in PATH.

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