plpgsql/parser: fix parsing for escaped strings

This commit adds support for scanning an escaped string constant in a
PL/pgSQL routine. This is necessary because escaped strings have imbalanced
quotes (like `e'foobar''`), which previously caused the scanner to return
an "unterminated string" error.

Informs #106368

Release note (bug fix): Fixed a bug that could result in a syntax error
if a PL/pgSQL routine was created with an escaped string constant in
the routine body. This bug has existed since v23.2.

pkg/sql/plpgsql/parser/testdata/const

@@ -1,3 +1,35 @@
+parse
+DECLARE
+  x STRING := e'1GW\'';
+BEGIN
+  INSERT INTO tab SELECT e'1GW\'';
+END
+----
+DECLARE
+x STRING := e'1GW\'';
+BEGIN
+INSERT INTO tab SELECT e'1GW\'';
+END;
+ -- normalized!
+DECLARE
+x STRING := (e'1GW\'');
+BEGIN
+INSERT INTO tab SELECT (e'1GW\'');
+END;
+ -- fully parenthesized
+DECLARE
+x STRING := '_';
+BEGIN
+INSERT INTO tab SELECT '_';
+END;
+ -- literals removed
+DECLARE
+_ STRING := e'1GW\'';
+BEGIN
+INSERT INTO _ SELECT e'1GW\'';
+END;
+ -- identifiers removed
+
 parse
 DECLARE
   x STRING := b'1GW\'';
@@ -94,27 +126,6 @@ INSERT INTO _ SELECT B'010101';
 END;
  -- identifiers removed
 
-# TODO(drewk): This is a valid constant.
-error
-DECLARE
-  x STRING := e'1GW\'';
-BEGIN
-  INSERT INTO tab SELECT e'1GW\'';
-END
-----
-at or near ";": at or near "EOF": syntax error
-DETAIL: source SQL:
-SET ROW (e'1GW\''
-                 ^
---
-source SQL:
-DECLARE
-  x STRING := e'1GW\'';
-BEGIN
-  INSERT INTO tab SELECT e'1GW\'';
-                                 ^
-HINT: try \h SET SESSION
-
 error
 DECLARE
   x STRING := 'foo;

pkg/sql/scanner/plpgsql_scan.go

@@ -70,6 +70,20 @@ func (s *PLpgSQLScanner) Scan(lval ScanSymType) {
 		s.scanIdent(lval)
 		return
 
+	case 'e', 'E':
+		// Escaped string?
+		if s.peek() == singleQuote {
+			// [eE]'[^']'
+			s.lastAttemptedID = int32(lexbase.SCONST)
+			s.pos++
+			if s.scanString(lval, singleQuote, true /* allowEscapes */, true /* requireUTF8 */) {
+				lval.SetID(lexbase.SCONST)
+			}
+			return
+		}
+		s.scanIdent(lval)
+		return
+
 	case '.':
 		switch t := s.peek(); {
 		case t == '.': // ..