Lua: tweaks and docs

common/space_lua/stdlib/string.ts

@@ -157,4 +157,12 @@ export const stringApi = new LuaTable({
   split: new LuaBuiltinFunction((_sf, s: string, sep: string) => {
     return s.split(sep);
   }),
+
+  // Non-standard
+  startswith: new LuaBuiltinFunction((_sf, s: string, prefix: string) => {
+    return s.startsWith(prefix);
+  }),
+  endswith: new LuaBuiltinFunction((_sf, s: string, suffix: string) => {
+    return s.endsWith(suffix);
+  }),
 });