Add Significant Text Aggregation

[russcam]

Introduced as an experimental feature in 6.0.

I think the usecases would warrant implementing this. Thoughts @Mpdreamz, @codebrain ?

[codebrain]

Seems reasonable - I imagine it'd just fit with the existing aggregations pattern in NEST?

[Mpdreamz]

Agreed, quite a few aggs were experimental for most of 2.x and 5.x 😄