Improved syntax highlighting support for Rust (#240)

Improved syntax highlighting support for Rust

The syntax highlighting for Rust [1] has been improved:

1. Use `nord7` as foreground color for entity types.
2. Use `nord8` as foreground color with bold font for macros [2] to
   make them visually different from "normal" functions.
3. Use `nord10` for attributes [3] and derives [4].
4. Use `nord13` for interpolation brackets to match the color for
   regular expression and special tokens in strings.
5. As a result of list item 1, import statements and paths are correctly
   colored with keyword and type colors.
6. Use the correct highlighting for traits [5] by making the font bold.

[1]: https://www.rust-lang.org
[2]: https://doc.rust-lang.org/1.8.0/book/macros.html
[3]: https://doc.rust-lang.org/reference/attributes.html
[4]: https://doc.rust-lang.org/book/ch19-06-macros.html#how-to-write-a-custom-derive-macro
[5]: https://doc.rust-lang.org/book/ch10-02-traits.html


Co-authored-by: Arctic Ice Studio <development@arcticicestudio.com>
Co-authored-by: Sven Greb <development@svengreb.de>

Resolves GH-167

Author: vhladko

themes/nord-color-theme.json

@@ -1215,6 +1215,46 @@
         "foreground": "#81A1C1"
       }
     },
+    {
+      "name": "[Rust] Entity types",
+      "scope": "source.rust entity.name.type",
+      "settings": {
+        "foreground": "#8FBCBB"
+      }
+    },
+    {
+      "name": "[Rust] Macro",
+      "scope": "source.rust meta.macro entity.name.function",
+      "settings": {
+        "fontStyle": "bold",
+        "foreground": "#88C0D0"
+      }
+    },
+    {
+      "name": "[Rust] Attributes",
+      "scope": [
+        "source.rust meta.attribute",
+        "source.rust meta.attribute punctuation",
+        "source.rust meta.attribute keyword.operator"
+      ],
+      "settings": {
+        "foreground": "#5E81AC",
+      }
+    },
+    {
+      "name": "[Rust] Traits",
+      "scope": "source.rust entity.name.type.trait",
+      "settings": {
+        "fontStyle": "bold"
+      }
+    },
+    {
+      "name": "[Rust] Interpolation Bracket Curly",
+      "scope": "source.rust punctuation.definition.interpolation",
+      "settings": {
+        "foreground": "#EBCB8B",
+      }
+    },
     {
       "name": "[SCSS] Punctuation Definition Interpolation Bracket Curly",
       "scope": [