diagnostics is slow

`diagnostics.html` while helpful is extremely slow.

to remedy that we should refactor the xquery code, move the bits to `app.xql` that belong there, and make use of range indexes for the numerous lookup and string comparisons that are going on. 


### Please provide the following
*   exist-db version: `5.0.0-SNAPSHOT`
*   documentation version: `4.1.3-SNAPSHOT`
