Enable anchoring at the start and end of fields for positional searching

[romseygeek]

Finding terms within X positions of the start of a field is currently possible via intervals queries, although it only really works for single-valued fields. It is not at all possible to find a term within X positions of the end of a field.

We should enable this kind of querying, either via access to text.length in interval scripts (not too hard to implement, but tricky to make it work correctly for multi-valued fields) or by indexing boundary markers in text fields.

[elasticmachine]

Pinging @elastic/es-search

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)