Fix highlighting of "injected language fragments" (#141)

Before the `INJECTED_LANGUAGE_FRAGMENT` editor color scheme key was
set to use `nord10` as foreground which overrides any language specific
highlighting of "injected language fragments", e.g. when using CSS-in-JS
libraries like styled-components [1] through the
"Styled Components & Styled JSX“ [2] plugin.

This has been fixed by removing the defined foreground color entirely so
the highlighting for the specific languages is uses instead.

[1]: https://styled-components.com
[2]: https://plugins.jetbrains.com/plugin/9997-styled-components--styled-jsx

Fixes GH-140

resources/themes/nord.xml

@@ -2186,9 +2186,7 @@
       </value>
     </option>
     <option name="INJECTED_LANGUAGE_FRAGMENT">
-      <value>
-        <option name="FOREGROUND" value="5e81ac" />
-      </value>
+      <value />
     </option>
     <option name="INLINE_PARAMETER_HINT">
       <value>