Syntax mappings for built-in code previewer

[anonymousgrasshopper]

bat has a --map-syntax feature which enables us to highlight some files without dedicated syntax highlighting as if they had another filetype, e.g. --map-syntax "*.asy:cpp", --map-syntax "*.conf:ini", or --map-syntax "*justfile:Makefile". It would be great if the built-in code previewer supported a similar feature as well; is there some way to achieve that (without completely overriding the previewer) ?

[sxyazi]

Duplicate of #1163, closing so we can track this in one place