[Discover] Handle text based (ES|QL) query checks consistently in Discover

[davismcphee]

Currently in Discover we use a few different methods of checking if a query is text based (ES|QL):

• getRawRecordType(query) === RecordRawType.PLAIN
• isTextBasedQuery(query)
• isOfAggregateQueryType(query)
• ...

We also use different variable names for the result which is confusing:

• isTextBased
• isPlainRecord
• ...

We should align on a single method of checking for text based / ES|QL queries and use consistent naming. Possibly something including ES|QL for clarity, along these lines: const isEsqlQuery = isEsqlQueryType(query).

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)