Feature: add Threepart LaTeX renderer

[jpweytjens]

Add ThreePartRenderer for improved LaTeX table output

Adds a new LaTeX renderer that produces publication-quality regression tables using threeparttable, booktabs, and siunitx LaTeX packages. This implementation offers several improvements over the standard LaTeX output:

• Uses threeparttable environment for better organization of table notes and star explanations
• Leverages booktabs package for professional-looking horizontal rules
• Implements siunitx for proper alignment of numerical values and significance stars
• Adds ability to generate standalone LaTeX documents with configurable preamble
• Includes a helper method to save tables directly to file

Key Features

• Proper alignment of coefficients and standard errors using siunitx's S column type
• Clean table rules from booktabs (toprule, midrule, bottomrule)
• Structured notes section under the table using threeparttable
• Optional standalone document generation with customizable packages and margins
• Direct file output support via save_to_file() method

The new renderer maintains full compatibility with existing Stargazer functionality while providing enhanced LaTeX output suitable for academic publications.

[jpweytjens]

An example of the LaTeXRenderer on the left versus the ThreepartRenderer on the right.

[toobaz]

Thanks @jpweytjens , looks great!

I will check more in depth next week.

[jpweytjens]

Hey @toobaz , have you found any time to have a look at this PR?