Add the regular expression functions that find each match

[tamnd]

Part of #3.

This change adds the regular expression functions that find each match in a string. The views of information_schema and the queries of client tools use some of them, such as regexp_replace. With this change, rupg has all the regular expression functions of PostgreSQL 19.

What changes

• Regex::find_chars finds a match from a given character. The constraints, such as ^ and a lookbehind, still see the characters before that place, as in pg_regexec.
• setup_matches ports setup_regexp_matches. It finds the first match or each match, and the search after an empty match starts one character later.
• regexp_replace has its five forms. The replacement takes \1 to \9, \& and \\, as appendStringInfoRegexpSubstr does. Options that start with a digit give the error of PostgreSQL with its hint.
• regexp_count, regexp_instr and regexp_substr have each of their forms, with the checks of start, n, endoption and subexpr.
• regexp_matches and regexp_split_to_table are set-returning functions in FROM. regexp_split_to_array gives the same parts as an array. An empty match at the end of the string or just after a match does not split the string.

Test

• The new session test regex_matches runs a script and compares the result with the output of PostgreSQL 19 for the same script in a new database. It covers each function and form, empty matches, a start after the first character, characters of more than one byte, LATERAL, each error and a view.
• srvtest passed on server1: fmt, layers, style, clippy, all the workspace tests, the generated file checks and the docs.