fix(latex): set utf-8 python io encoding for windows

[AtifChy]

Summary

Fixes LaTeX rendering on Windows when latex2text emits Unicode math symbols that cannot be represented by the active Windows code page.

The converter now runs with PYTHONIOENCODING=utf-8, which prevents Python-based latex2text executables from failing with encoding errors when stdout is redirected.

Before	After

Details

• Sets PYTHONIOENCODING=utf-8 for the vim.system converter path.
• Temporarily sets and then restores vim.env.PYTHONIOENCODING for the legacy vim.fn.system fallback.
• Preserves any existing user environment value so the plugin does not leak global environment changes into later processes.
• Adds test coverage to ensure the modern converter path passes the UTF-8 environment.

Verification

• Confirmed the failing sample now converts successfully on Windows.
• All tests passed.

[AtifChy]

@MeanderingProgrammer this is ready for merging.