Determine if there are index size enhancements possible if `excerpts_per_result` is 0

[jameslittle230]

https://stork-search.net/docs/config-ref#excerpts_per_result

[jameslittle230]

There are!

• If excerpts_per_result is 0, we can write 0 excerpts to each result's excerpts vec
• If excerpts_per_result is 0, we can write the result's contents as an empty string

Note that we can't only write excerpts_per_result excerpts to each result's excerpts vec, because scoring those excerpts only takes place during search, so we don't know which ones we can shed at build time.