Use a phrase suggester for spellcheck.

The existing spellcheck functionality provided by this module works well for single words, but for phrases, can produce some weird results.

For example, on my client's site, a search for "summer session" will return results in the search, and also offer "Did you mean: suffer session?". "virgin active pools" will offer "Did you mean: vermin action pool?". This could be better...

I' ve been working on setting up a phrase suggester, as per the opensearch docs: https://docs.opensearch.org/docs/latest/search-plugins/searching-data/di...

So far, this seems to work much better than the existing setup. I've set up an analyser, data type and mapping so far, which was enough to fire requests at Opensearch with curl to evaluate it.

Still to do is the request side of things, and test coverage.