Add `string_content` node for easier querying

[alexmozaidze]

Made colon in a colon string to be its own anonymous node, and added (string_content) which captures just the string's content, without the colon or quotes, making it easier to query:

:my-string

---

(string
  ":"
  (string_content))

[!IMPORTANT]
There is an unpleasant side-effect of a single colon : being detected as a string with missing string_content. This is not a big issue, since the only place a single colon is used is in a binding, which itself overrides (string). A more elegant solution would be an external parser, but that's too much for such a small issue.