regex: fix find() when using anchors (start / end) (#18259)

vlib/regex/regex_anchor_test.v

@@ -0,0 +1,22 @@
+import regex
+
+fn test_anchor_start() {
+	mut re := regex.regex_opt(r'^\w+') or { panic(err) }
+	start, end := re.find('id.')
+	assert start == 0
+	assert end == 2
+}
+
+fn test_anchor_end() {
+	mut re := regex.regex_opt(r'\w+$') or { panic(err) }
+	start, end := re.find('(id')
+	assert start == 1
+	assert end == 3
+}
+
+fn test_anchor_both() {
+	mut re := regex.regex_opt(r'^\w+$') or { panic(err) }
+	start, end := re.find('(id)')
+	assert start == -1
+	assert end == -1
+}