feat: add std.isEmpty

Upstream issue: google/go-jsonnet#678

Author: pawelbeza

crates/jrsonnet-stdlib/src/lib.rs

@@ -148,6 +148,7 @@ pub fn stdlib_uncached(settings: Rc<RefCell<Settings>>) -> ObjValue {
 		("substr", builtin_substr::INST),
 		("char", builtin_char::INST),
 		("strReplace", builtin_str_replace::INST),
+		("isEmpty", builtin_is_empty::INST),
 		("splitLimit", builtin_splitlimit::INST),
 		("asciiUpper", builtin_ascii_upper::INST),
 		("asciiLower", builtin_ascii_lower::INST),

crates/jrsonnet-stdlib/src/strings.rs

@@ -27,6 +27,11 @@ pub fn builtin_str_replace(str: String, from: IStr, to: IStr) -> String {
 	str.replace(&from as &str, &to as &str)
 }
 
+#[builtin]
+pub fn builtin_is_empty(str: String) -> bool {
+	str.is_empty()
+}
+
 #[builtin]
 pub fn builtin_splitlimit(str: IStr, c: IStr, maxsplits: Either![usize, M1]) -> ArrValue {
 	use Either2::*;