Fix parsing of _ENV

examples/test.lua

@@ -93,3 +93,5 @@ end
 for k, v in (ipairs(table)) do
     print(k, v)
 end
+
+print(_ENV.sometable)

src/token_stream/lexer.rs

@@ -286,7 +286,7 @@ pub fn tokenize<T: Into<String>>(to_tokenize: T) -> Result<Vec<FullToken>, Error
                 }
             }
             // "words"
-            c if c.is_alphabetic() => {
+            c if c.is_alphabetic() || *c == '_' => {
                 let mut value = character.to_string();
                 while let Some(c) = cursor.first() {
                     if !(c.is_ascii_alphanumeric() || c == '_') {