feat: support .py, .R, .jl files in QuartoPreview

[Copilot]

QuartoPreview was silently rejecting script files (.py, .R, .jl) that quarto preview supports natively, including Python files using jupytext format.

Change

• Extended quarto_extensions allowlist in quartoPreview to include .py, .R, and .jl

-- before
local quarto_extensions = { '.qmd', '.Rmd', '.ipynb', '.md' }

-- after
local quarto_extensions = { '.qmd', '.Rmd', '.ipynb', '.md', '.py', '.R', '.jl' }