feat/pager: syntax-rendering in pager

[dextermallo]

Description

Hi Charm team, love the ideas of gum and other handy utilities!
I have an idea for the command pager: syntax-highlight when outputting the contents. (which is one of my use cases)

Describe the solution you'd like

1. I am unsure whether it is applicable to introduce other packages like alecthomas/chroma to do the full syntax highlight on every type of file, so I use glamour to do it first to see if the idea is applicable.
2. Followed by 1, it only supports with --syntax-highlight=markdown now.

• Add a flag --syntax-highlight=<language> under /pager/options.go

Example

I am quite new to Go but would love to do on other languages as well :)