EQL: Improve handling for non-keyword field exact matching

[aleksmaus]

The error messaging could be improved, which is currently:
"1:11: [multi_field.english == 'bar'] cannot operate on first argument field of data type [text]: No keyword/multi-field defined exact matches for [english]; define one or use MATCH/QUERY instead"

Need to discuss "to get consensus about how we deal with exact matches on analyzed fields - whether we prevent any exact comparison against them or introduce some other type of comparison (inexact matches - say ~=)"

This is related to the PR #52664

Discuss ...

[elasticmachine]

Pinging @elastic/es-search (:Search/EQL)