[psteinroe]

Stacked on #918.

Two cases #918 left out: function bodies written as escape strings, and the PL/pgSQL body range, which still used content.find.

create function escaped_fn() returns text language sql
as E'select \'é\' || name from users where gone = 1';
-- before: no tokens, hints, hover, or diagnostics in the body
-- after:  name and users get tokens, and unknownColumn is on `gone`

create function f() returns void language plpgsql
as 'begin raise notice ''x''; ende';
-- before: no PL/pgSQL syntax error at all (the body text wasn't found in the source)
-- after:  the syntax error covers the body

What changed

• StatementOffsets (pgls_analyser) maps escapes of any length instead of only '': each escape records the bytes it stands for in the statement and its bytes in the document. Offsets inside an escape map to its start in both directions.
• sql_function::string_body unescapes '...' and E'...' constants after AS, following the xe rules of Postgres' lexer (scan.l#L262 at REL_18_6): \b \f \n \r \t \v, octal, \x hex, \u/\U, '', and \ before any other character. A value that isn't valid UTF-8 (e.g. \xff), a \u without four digits, a surrogate pair, or a constant continued on a new line is not matched, and the body gets no statements like before. U&'...' bodies are still not supported.
• PgQueryStore::get_or_cache_plpgsql_parse uses string_body for the range of PL/pgSQL syntax errors. If the body can't be located, the error now covers the whole statement instead of the check being skipped.

Tests

• unescape unit tests: every escape kind, multi-byte UTF-8 from \x pairs, and the cases that aren't matched.
• StatementOffsets: an escape that is longer than the bytes it stands for.
• sql_function: an E'...' body with '', \', \\, \n and é, mapped back to the document.
• pg_query: PL/pgSQL syntax error spans for a body with '' and for a body whose text also appears earlier, in a parameter default.
• Workspace: tokens and an unknownColumn diagnostic in an E'...' body.

Not covered

U&'...' bodies; PL/pgSQL bodies still get no editor features (only the syntax check).