feat: allow underscores in named capturing groups

Author: dangh

grammar.js

@@ -219,7 +219,7 @@ module.exports = grammar({
     //   [+U]uNonSurrogate
     //   [~U]uHex4Digits
     //   [+U]u{CodePoint}
-    group_name: _ => /[A-Za-z0-9]+/,
+    group_name: _ => /[A-Za-z_][A-Za-z0-9_]*/,
 
     decimal_digits: _ => /\d+/,
   },

src/grammar.json

@@ -452,7 +452,7 @@ hello (.*)
 ================================================================================
 named capturing groups
 ================================================================================
-(?<world>.*)
+(?<the_world>.*)
 --------------------------------------------------------------------------------
 
 (pattern
@@ -467,7 +467,7 @@ named capturing groups
 ================================================================================
 named capturing groups - backreferences
 ================================================================================
-(?<world>.*)\kworld
+(?<the_world>.*)\kthe_world
 --------------------------------------------------------------------------------
 
 (pattern