Follow relative links in TUI mode

[aleexharris]

Already long-standing discussion of this as a potential feature in #441

I've scoped the feature as the following:

• Add TUI pager support for following relative links to other local markdown files.
• Use Tab/Shift+Tab to select, Enter to follow links and Backspace to go back to parent.
• Only follow relative links that resolve to an existing regular file within the directory root Glow was opened in.

I've produced an implementation here: #883

[c-nv-s]

I was hoping this feature would already be in Glow, it seems like a very sensible expectation to me.
As well as a tree view navigation pane for files e.g. https://github.com/Epistates/treemd/blob/main/assets/output.gif