feat(lexer): add token definitions

src/main/kotlin/lumen/lexer/Token.kt

@@ -0,0 +1,64 @@
+package lumen.lexer
+
+enum class TokenType {
+    // literals
+    IntLiteral,
+    DoubleLiteral,
+    StringLiteral,
+    Identifier,
+
+    // keywords
+    Let,
+    Var,
+    Fn,
+    Return,
+    If,
+    Else,
+    While,
+    For,
+    In,
+    True,
+    False,
+    Nil,
+    And,
+    Or,
+    Not,
+    Import,
+
+    // punctuation
+    Plus,
+    Minus,
+    Star,
+    Slash,
+    Percent,
+    Assign,
+    EqualEqual,
+    BangEqual,
+    Less,
+    LessEqual,
+    Greater,
+    GreaterEqual,
+    LeftParen,
+    RightParen,
+    LeftBracket,
+    RightBracket,
+    LeftBrace,
+    RightBrace,
+    Comma,
+    Colon,
+    Semicolon,
+    Dot,
+    Newline,
+
+    Eof
+}
+
+data class Token(
+    val type: TokenType,
+    val lexeme: String,
+    val line: Int,
+    val column: Int = 1,
+    val intValue: Long = 0L,
+    val doubleValue: Double = 0.0,
+    val stringValue: String = ""
+)