Add expressions inside string attribute values

[savetheclocktower]

Fixes #44.

I tried to roughly follow the design of template strings in tree-sitter-javascript. Instead of doing something like…

(attribute_value
  (value_fragment)
  (expression (raw_text_expr))
  (value_fragment)
)

…I instead did…

(attribute_value
  (expression (raw_text_expr))
)

…so if you want to identify just the literal string parts, you can take (attribute_value) and ignore/subtract its children.

[savetheclocktower]

By the way, I've purposefully been making atomic PRs, but I've also got a branch with all of these changes combined. If they all look good to you individually, I can open a PR against that branch and we can save ourselves some merging headaches.

[Himujjal]

@savetheclocktower I would love for you to make a PR for the same. It would save us time as well