feat: add NATS config file lexer (port of Go conf/lex.go)

Port the NATS configuration file lexer from Go's conf/lex.go to C#.
The lexer is a state-machine tokenizer that supports the NATS config
format: key-value pairs with =, :, or whitespace separators; nested
maps {}; arrays []; single and double quoted strings with escape
sequences; block strings (); variables $VAR; include directives;
comments (# and //); booleans; integers with size suffixes (kb, mb, gb);
floats; ISO8601 datetimes; and IP addresses.

src/NATS.Server/Configuration/NatsConfToken.cs

@@ -0,0 +1,24 @@
+// Port of Go conf/lex.go token types.
+
+namespace NATS.Server.Configuration;
+
+public enum TokenType
+{
+    Error,
+    Eof,
+    Key,
+    String,
+    Bool,
+    Integer,
+    Float,
+    DateTime,
+    ArrayStart,
+    ArrayEnd,
+    MapStart,
+    MapEnd,
+    Variable,
+    Include,
+    Comment,
+}
+
+public readonly record struct Token(TokenType Type, string Value, int Line, int Position);