Fixes#2500
Based on #2508
This separates out the concept of a "hyperlink" from a "hyperlink page
entry." The difference is that the former has real Zig slices into
things like strings and the latter has offsets into terminal page
memory.
From this separation, the Page structure now has an `insertHyperlink`
function that takes a hyperlink and converts it to a page entry.
This does a couple things: (1) it moves page memory management out of
Screen and into Page which is historically more appropriate and (2) it
let's us more easily test hyperlinks from the Page unit tests.
Finally, this PR makes some error sets explicit.
6109edd00b