fixes: dolthub/dolt#9039

Inserts nested inside triggers is a special case that we have minimal testing for. Some of the things that were broken before:

• we did not prepend nodes inside trigger->insert blocks, so new columns were not accessible / the index offsets estimated based on prepending trigger rows were invalid
• subqueries inside trigger->insert that have no outer visibility did not make exceptions for trigger rows, which act like lateral rows
• prepending rules are flaky and mistakenly prepended in-between project->project nodes