Search: Requirements for replacement search functionality

The following requests have been made in other issues; I am closing those issues in favour of this super-issue for considerations when selecting a replacement search system. Note that the migration to Zensical might provide some of these by default.

Desired features/functionality:

- ability to search for multi-word terms specified within double quotes
user: "Searching for multi-word terms such as "iota underbar" does not return helpful results (it does not return the page for the where function or interval index function, or even the disambiguation page for the glyph) but rather returns any page that includes either term."
- search functionality that includes logic options such as AND, OR, and NOT.
- allow case-sensitive searching and exact matches to be toggled.
- results should prioritise exact matches
user: "I searched for ⎕R and the results showed ⎕REFS (1 page), ⎕RSI, ⎕RTL, ⎕RL on the next page and finally ⎕R. It would be nice if ⎕R was at the top of the list - since it is an exact word match, not a partial one like the others."
- results should indicate the document in which the result has been found
Either the breadcrumb trail of the URL