Use correct icons for each file type in the Repository Browser tree

Problem to solve

As we're navigating the tree of a repository, we show a generic file icon.

Currently	Proposed

Further details

It's not always easy to easily scan the file list and identify the file we're looking for. If we have the icon, it'll be easy to filter out a certain file type.

Proposal

We have all the information necessary to guess the icon based on the file extension.

We can also reuse the logic we have in the MR Diff file header:

It could look something like this:

Documentation

We'll most likely have to update screenshots in the docs to show the new icons.

What does success look like, and how can we measure that?

Faster for users to find the desired file on a long list.

Links / references