Allow index sorting by field length [LUCENE-8069]

Short documents are more likely to get higher scores, so sorting an index by field length would mean we would be likely to collect best matches first. Depending on the similarity implementation, this might even allow to early terminate collection of top documents on term queries.

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Migrated from [LUCENE-8069](https://issues.apache.org/jira/browse/LUCENE-8069) by Adrien Grand (@jpountz), updated Apr 27 2021

The issue names no files, tests, or entry points. Begin by mapping Lucene's index-sorting and similarity or term-query collection paths, then define and test what field-length sorting and any early-termination behavior should mean.

Written by the indexing model from the issue text.