schemas/util.sql: added schema_regexp(schema_anchor)

fix: schemas/vegbien.sql: _plots_04_count_of_plots_in_each_project_in_this_source: count top-level plots to match the input query

schemas/vegbien.sql: remake_diff_table(): instructions for how to regenerate the diff table: put these at the beginning of the table comment instead of the end, because they are the most important info about the table that a user needs to know

schemas/util.sql: diff(text, text): documented how to write a custom keys() function to match up rows using a subset of the columns (including a sample keys() function template)

schemas/vegbien.sql: replaced custom %== operators with much simpler custom keys() functions. this avoids both the need to write out an = comparison for each field, and especially, also the need to write both an operator and a function implementing that operator.

bugfix: schemas/util.sql: diff(text, text): in the %== comparison, it turns out you do need to cast the values to the same base type, even though this is optional when using a custom %==

schemas/util.sql: remake_diff_table(): prepend the "view this table in human-readable form" comment instead of appending because it is more important than the "contents generated from" comment previously added

schemas/util.sql: added prepend_comment()

schemas/util.sql: removed no longer used mk_set_search_path(VARIADIC schemas text[]). use mk_search_path() instead.

schemas/util.sql, vegbien.sql: comment about "function option search_path": don't include `mk_set_search_path()` since that is no longer used