vfmt: fix eating `c` in `c'foo'` (#7009)

* fmt: add formatting for cstrs

* fmt

vlib/v/fmt/fmt.v

@@ -1031,6 +1031,8 @@ pub fn (mut f Fmt) expr(node ast.Expr) {
 		ast.StringLiteral {
 			if node.is_raw {
 				f.write('r')
+			} else if node.language == table.Language.c {
+				f.write('c')
 			}
 			if node.val.contains("'") && !node.val.contains('"') {
 				f.write('"$node.val"')

vlib/v/fmt/tests/string_raw_and_cstr_keep.vv

@@ -0,0 +1,6 @@
+fn main() {
+	raw := r'\x00'
+	cstr := c'foo'
+	println(raw)
+	println(cstr)
+}