Add Syntax Preview for other types of files

[JordyScript]

Currently, one can only check how the selected colors work on Markdown syntax, but it would be nice if the theme could be previewed on other file types, like source code files in various languages.

[kmwallio]

Thank you for the feedback! I'll work on adding a drop down to change the language preview.

[kmwallio]

Just as an update, I'm pushing a new version with GTK4 and Language Selection support. I added Python, C, Rust, C#, HTML, and some others. Please let me know if there's some additional languages you'd like to see.

For those familiar with GtkSourceView style schemes, hovering over the Markdown label will show the corresponding style it will apply to.

Later on, I'm hoping I can get the labels to match the language selected, but that's a bigger effort.