# Description

This PR adds support for injected languages using tree-sitter as
described in #16 and in the [tree-sitter
documentation](https://tree-sitter.github.io/tree-sitter/syntax-highlighting#language-injection).
Languages can contain _injected_ languages which are described in a
language's `injections.scm` file. Some examples of injected languages
are:
- HTML contains CSS and Javascript in `style` and `script` tags
- Javascript contains Regex literals
- PHP contains HTML between the `<php` tags
- C++ can contain rawstring literals of arbitrary languages

# Details

This PR is a rework of the `TreeSitterClient` class. Specifically it:
- Adds a `layers` array and `primaryLayer` property. `layers` contains
all language layers in the document, and `primaryLayer` is the ID of the
document's primary language.
- Each layer is a `LanguageLayer` object. These objects represent an
injected language-range(s) combination. Each layer can have one or more
range associated with it depending on if it should be [parsed as one
document or
multiple](https://tree-sitter.github.io/tree-sitter/syntax-highlighting#language-injection).
- When editing:
- Each layer's ranges are updated using a similar algorithm to
tree-sitter, and edits are applied to make use of the incremental
parsing tree-sitter gives.
- Each layer is checked for any injections, inserting any new injected
layers and keeping track of any 'untouched' layers to remove.
  - Any layers that were not touched are removed.

The highlight query algorithm is largely the same, but keeps track of
any ranges not used by any injected layers, and only queries the primary
layer for those ranges so as not to override any injected highlights.

# Related Issues

- Closes #16 

# Screenshots

Before, other languages were detected but parsed and highlighted as
normal text.
<img width="1104" alt="Screenshot 2023-02-28 at 3 08 52 PM"
src="https://user-images.githubusercontent.com/35942988/221980502-3aa61b6a-136a-43b9-a545-8fd835945002.png">

With Injected languages, in this case CSS and JS embedded in HTML and a
second layer of Regex embedded in JS embedded in HTML:
<img width="889" alt="Screenshot 2023-03-24 at 2 45 15 PM"
src="https://user-images.githubusercontent.com/35942988/227628557-df55d986-a104-4a24-ab6c-97c8e69f6136.png">