add language grammar for Isabelle theorem prover

grammars.yml

@@ -408,3 +408,5 @@ https://github.com/vmg/zephir-sublime:
 - source.php.zephir
 https://github.com/whitequark/llvm.tmbundle:
 - source.llvm
+https://github.com/lsf37/Isabelle.tmbundle:
+- source.isabelle.theory

lib/linguist/languages.yml

@@ -1211,7 +1211,7 @@ Isabelle:
   color: "#fdcd00"
   extensions:
   - .thy
-  tm_scope: none
+  tm_scope: source.isabelle.theory
 
 J:
   type: programming