feat: add text nodes (#3)

- `text` nodes are now shown in the syntax tree
- `text` nodes include whitespace characters but not linebreak characters
- whitespace characters that are used for defining block structure are not part of `text` nodes
- `text` node positions are accurate so that we could grab their value contents easily

README.md

@@ -35,12 +35,15 @@ console.log(tree.rootNode.toString());
 // (document
 //   (atx_heading
 //     (atx_heading_marker)
-//     (heading_content))
+//     (heading_content
+//       (text)))
 //   (tight_list
 //     (list_item
 //       (list_marker)
-//       (indented_code_block)
-//       (paragraph))))
+//       (indented_code_block
+//         (text))
+//       (paragraph
+//         (text)))))
 ```
 
 ## License