v.util: fix handling of "start \u005c${...} end" in cgen (fix #25304) (#25306)

vlib/v/gen/c/testdata/string_escapes_and_quoting_issue_25304_1.out

@@ -0,0 +1,2 @@
+start
+done

vlib/v/gen/c/testdata/string_escapes_and_quoting_issue_25304_1.vv

@@ -0,0 +1,6 @@
+println('start')
+b := 'Hello world!\u004azz\u005cXYZ'
+a := 'zzz\u005c${')}}));builtin__println(b);//'}QWE'
+_ := a
+_ := b
+println('done')

vlib/v/gen/c/testdata/string_escapes_and_quoting_issue_25304_2.out

@@ -0,0 +1,2 @@
+start
+done

vlib/v/gen/c/testdata/string_escapes_and_quoting_issue_25304_2.vv

@@ -0,0 +1,6 @@
+println('start')
+b := 'Hello world!'
+c := '\u005c${')}}));builtin__println(b);//'}'
+_ := b
+_ := c
+println('done')

vlib/v/util/quote.v

@@ -114,6 +114,10 @@ pub fn smart_quote(str string, raw bool) string {
 			skip_next = true
 			continue
 		}
+		// protect against '\u005c${...}'
+		if next == 0 && current == backslash {
+			continue
+		}
 		if !raw {
 			if current == `$` {
 				if last == backslash {