Backslash at the end of string causes wrong coloring

[yuriever]

Thanks for the useful highlighter!

The minimal example is

``` wl
"\"a
```

which will be rendered as a whole string instead of string with the rest expression,

<div class="highlight"><pre id="__code_3"><span></span><code><span class="s">"\"a</span></code></pre></div>

Only after inserting some character to avoid escaping we can get the expected coloring,

``` wl
"\ "a
```

<div class="highlight"><pre id="__code_3"><span></span><code><span class="s">"\ "</span><span class="nv">a</span>
</code></pre></div>

Could you fix this issue?