fix(tokenizer): enter ident mode when appropriate

There was a bug brought up in #54
where the tokenizer incorrectly enters identifier mode after seeing a `when/given`.

This means that `bulloak` failed to scaffold the following:

```tree
Foo
└── It reverts when X.
```

Because it saw a `.` after `when`, even though this is a perfectly valid
tree.

We fix this by checking that `when/given` only make the tokenizer enter
identifier mode if the last processed token was a branching token
(either `Corner` or `Tee` currently).

src/syntax/parser.rs

@@ -789,6 +789,27 @@ mod tests {
         );
     }
 
+    // https://github.com/alexfertel/bulloak/issues/54
+    #[test]
+    fn parses_top_level_actions() {
+        assert_eq!(
+            parse(
+                r#"Foo
+└── It reverts when X."#
+            )
+            .unwrap(),
+            Ast::Root(Root {
+                contract_name: String::from("Foo"),
+                span: s(p(0, 1, 1), p(31, 2, 22)),
+                children: vec![Ast::Action(Action {
+                    title: String::from("It reverts when X."),
+                    span: s(p(4, 2, 1), p(31, 2, 22)),
+                    children: vec![]
+                })],
+            })
+        );
+    }
+
     #[test]
     fn unsanitized_input() {
         assert_eq!(

src/syntax/tokenizer.rs

@@ -83,6 +83,15 @@ pub struct Token {
     pub lexeme: String,
 }
 
+impl Token {
+    fn is_branch(&self) -> bool {
+        match self.kind {
+            TokenKind::Tee | TokenKind::Corner => true,
+            TokenKind::Word | TokenKind::When | TokenKind::Given | TokenKind::It => false,
+        }
+    }
+}
+
 impl fmt::Debug for Token {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(
@@ -329,7 +338,10 @@ impl<'s, T: Borrow<Tokenizer>> TokenizerI<'s, T> {
                 }
                 _ => {
                     let token = self.scan_word()?;
-                    if token.kind == TokenKind::When || token.kind == TokenKind::Given {
+                    let last_is_branch = tokens.last().is_some_and(Token::is_branch);
+                    if last_is_branch
+                        && (token.kind == TokenKind::When || token.kind == TokenKind::Given)
+                    {
                         self.enter_identifier_mode();
                     };
                     tokens.push(token);