Add language definition for Ur/Web

I have omitted the related .urp extension because these are small
files (projects) in a different syntax.

lib/linguist/languages.yml

@@ -3475,6 +3475,17 @@ UnrealScript:
   tm_scope: source.java
   ace_mode: java
 
+UrWeb:
+  type: programming
+  aliases:
+  - Ur/Web
+  - Ur
+  extensions:
+  - .ur
+  - .urs
+  tm_scope: source.ur
+  ace_mode: text
+
 VCL:
   group: Perl
   type: programming