Add textutils.unserialiseJSON (#407)

This is relatively unoptimised right now, but should be efficient enough
for most practical applications.

 - Add textutils.json_null. This will be serialized into a literal
   `null`. When deserializing, and parse_null is true, this will be
   returned instead of a nil.

 - Add textutils.unserializeJSON (and textutils.unserializeJSON). This
   is a standard compliant JSON parser (hopefully).

 - Passing in nbt_style to textutils.unserializeJSON will handle
   stringified NBT (no quotes around object keys, numeric suffices). We
   don't currently support byte/long/int arrays - something to add in
   a future commit.

tools/check-lines.py

@@ -3,10 +3,10 @@ import pathlib, sys
 problems = False
 
 # Skip images and files without extensions
-exclude = [ ".png", "" ]
+exclude = [ "*.png", "**/data/json-parsing/*.json" ]
 
-for path in pathlib.Path(".").glob("src/**/*"):
-    if path.is_dir() or path.suffix in exclude:
+for path in pathlib.Path("src").glob("**/*"):
+    if path.is_dir() or path.suffix == "" or any(path.match(x) for x in exclude):
         continue
 
     with path.open(encoding="utf-8") as file: