highlighting of multiline strings treats any `"""` as the end of the string instead of ending a multi-line string on a `"""` that only has whitespace proceeding it

Example:

```
defmodule Foo do
@moduledoc """
Whoops
Something
Bar"""

@it true

@doc """correct
"""
def baz, do: @it
end
```

Currently shows highlighting that indicates the `moduledoc` multi-line string stops at `Bar"""` in highlighting, but in fact it ends the line before `def baz` at the `"""` on that line.

This is _probably_ because the syntax file is missing the `^` anchor for the `end` attribute: https://github.com/elixir-lsp/vscode-elixir-ls/blob/d60e7b7710e3861f02cdfbdfc81297b2a638f108/syntaxes/elixir.json#L31-L44

This is potentially an issue for some other cases of `"""`, in that all the `end` attributes should likely have the `^` anchor to ensure the entire proceeding line is empty. It _appears_ that only the 2 `moduledoc` entries are missing the `^`.