sql/parser: fix minor issue with recent LTREE additions

We recently added parser support for FIRST_CONTAINS and
FIRST_CONTAINED_BY operators (`?@>` and `?<@` respectively). The AI
review pointed out a minor bug in how we handle these operators in the
scanner - we incorrectly advanced the position unconditionally before
matching the following characters. Instead, we need to use `peekN` to
look a few characters ahead to see whether we have a match, and this
commit does that. Note that the impact of the bug is very minor - we
simply would put the `^` character in the error in the wrong position.

Note that two newly-added tests have slightly different behavior with
regards to the `^` character which seems to be due to peculiarities of
the tokenizing / parsing that I didn't look closely into.

Additionally, this commit adds a newline character in an unrelated
parser test file.

Release note: None

Author: yuzefovich

pkg/sql/parser/testdata/changefeed

@@ -177,4 +177,4 @@ CREATE CHANGEFEED FOR database INTO 'foo'
 at or near "into": syntax error
 DETAIL: source SQL:
 CREATE CHANGEFEED FOR database INTO 'foo'
-                               ^
+                               ^

pkg/sql/parser/testdata/select_exprs

@@ -2171,10 +2171,26 @@ SELECT (((ARRAY[('A.B'), ('')])::LTREE[]) ?@> (('A')::LTREE)) -- fully parenthes
 SELECT ARRAY['_', '_']::LTREE[] ?@> '_'::LTREE -- literals removed
 SELECT ARRAY['A.B', '']::LTREE[] ?@> 'A'::LTREE -- identifiers removed
 
+error
+SELECT ARRAY['A.B', '']::LTREE[] ?@< 'A'::LTREE
+----
+at or near "<": syntax error
+DETAIL: source SQL:
+SELECT ARRAY['A.B', '']::LTREE[] ?@< 'A'::LTREE
+                                   ^
+
 parse
 SELECT ARRAY['A.B', '']::LTREE[] ?<@ 'A'::LTREE
 ----
 SELECT ARRAY['A.B', '']::LTREE[] ?<@ 'A'::LTREE
 SELECT (((ARRAY[('A.B'), ('')])::LTREE[]) ?<@ (('A')::LTREE)) -- fully parenthesized
 SELECT ARRAY['_', '_']::LTREE[] ?<@ '_'::LTREE -- literals removed
 SELECT ARRAY['A.B', '']::LTREE[] ?<@ 'A'::LTREE -- identifiers removed
+
+error
+SELECT ARRAY['A.B', '']::LTREE[] ?<< 'A'::LTREE
+----
+at or near "<": syntax error
+DETAIL: source SQL:
+SELECT ARRAY['A.B', '']::LTREE[] ?<< 'A'::LTREE
+                                  ^

pkg/sql/scanner/scan.go

@@ -311,18 +311,16 @@ func (s *SQLScanner) Scan(lval ScanSymType) {
 			lval.SetID(lexbase.JSON_ALL_EXISTS)
 			return
 		case '@':
-			s.pos++
-			switch s.peek() {
+			switch s.peekN(1) {
 			case '>': // ?@>
-				s.pos++
+				s.pos += 2
 				lval.SetID(lexbase.FIRST_CONTAINS)
 				return
 			}
 		case '<':
-			s.pos++
-			switch s.peek() {
+			switch s.peekN(1) {
 			case '@': // ?<@
-				s.pos++
+				s.pos += 2
 				lval.SetID(lexbase.FIRST_CONTAINED_BY)
 				return
 			}