Fix for C# identifiers with leading underscores

Author: chalin

lexers/embedded/csharp.xml

@@ -49,7 +49,7 @@
       <rule pattern="&#39;\\.&#39;|&#39;[^\\]&#39;">
         <token type="LiteralStringChar"/>
       </rule>
-      <rule pattern="0[xX][0-9a-fA-F]+[Ll]?|[0-9_](\.[0-9]*)?([eE][+-]?[0-9]+)?[flFLdD]?">
+      <rule pattern="0[xX][0-9a-fA-F]+[Ll]?|\d[_\d]*(\.\d*)?([eE][+-]?\d+)?[flFLdD]?">
         <token type="LiteralNumber"/>
       </rule>
       <rule pattern="#[ \t]*(if|endif|else|elif|define|undef|line|error|warning|region|endregion|pragma|nullable)\b[^\n\r]+">

lexers/testdata/csharp/csharp_var.actual

@@ -0,0 +1 @@
+var _abc = 123;

lexers/testdata/csharp/csharp_var.expected

@@ -0,0 +1,11 @@
+[
+  {"type":"KeywordType","value":"var"},
+  {"type":"Text","value":" "},
+  {"type":"Name","value":"_abc"},
+  {"type":"Text","value":" "},
+  {"type":"Punctuation","value":"="},
+  {"type":"Text","value":" "},
+  {"type":"LiteralNumber","value":"123"},
+  {"type":"Punctuation","value":";"},
+  {"type":"Text","value":"\n"}
+]