cgen: % escape in string interpolated literals

* cgen: % escape in c call to _STR
* tests: add test for string % escape
* Add a test for the % escaping when interpolating inside strings

vlib/v/gen/cgen.v

@@ -2363,7 +2363,7 @@ fn (g mut Gen) string_inter_literal(node ast.StringInterLiteral) {
 	g.write('_STR("')
 	// Build the string with %
 	for i, val in node.vals {
-		escaped_val := val.replace_each(['"', '\\"', '\r\n', '\\n', '\n', '\\n'])
+		escaped_val := val.replace_each(['"', '\\"', '\r\n', '\\n', '\n', '\\n', '%', '%%'])
 		g.write(escaped_val)
 		if i >= node.exprs.len {
 			continue

vlib/v/tests/inout/string_interp.out

@@ -0,0 +1 @@
+%.*sworldhello

vlib/v/tests/inout/string_interp.vv

@@ -0,0 +1,6 @@
+
+fn main() {
+	test := 'hello'
+	hello := 'world'
+    println('%.*s$hello$test')
+}

vlib/v/tests/string_interpolation_test.v

@@ -55,3 +55,10 @@ fn test_implicit_str() {
 	text := '$i' + '42'
 	assert text == '4242'
 }
+
+fn test_string_interpolation_percent_escaping(){
+	test := 'hello'
+	hello := 'world'
+	x := '%.*s$hello$test |${hello:-30s}|'
+	assert x == '%.*sworldhello |world                         |'
+}