TextMate rules for Markdown math formula syntax highlighting (#222)

VS Code 1.58 [1] (June 2021) introduced syntax highlighting for Markdown
math formulas [2]. To adapt the coloring to Nord's style new TextMate
rules with the following scopes have been added:

- `text.html.markdown constant.character.math.tex` - targets constants
  like `\displaystyle` and `\left`.
- `text.html.markdown constant.character.math.tex` - targets `$$`
  definition markers.
- `text.html.markdown punctuation.definition.function.math.tex` -
  targets the `\` (backslash) function definition marker.
- `text.html.markdown punctuation.math.operator.latex` - targets
  operators like `=`.

[1]: https://code.visualstudio.com/updates/v1_58
[2]: https://code.visualstudio.com/updates/v1_58#_markdown-math-formula-syntax-highlighting

Co-authored-by: Sven Greb <development@svengreb.de>

Closes GH-221

themes/nord-color-theme.json

@@ -1095,6 +1095,37 @@
         "foreground": "#616E88"
       }
     },
+    {
+      "name": "[Markdown] Markup Math Constant",
+      "scope": "text.html.markdown constant.character.math.tex",
+      "settings": {
+        "foreground": "#81A1C1"
+      }
+    },
+    {
+      "name": "[Markdown] Markup Math Definition Marker",
+      "scope": [
+        "text.html.markdown punctuation.definition.math.begin",
+        "text.html.markdown punctuation.definition.math.end"
+      ],
+      "settings": {
+        "foreground": "#5E81AC"
+      }
+    },
+    {
+      "name": "[Markdown] Markup Math Function Definition Marker",
+      "scope": "text.html.markdown punctuation.definition.function.math.tex",
+      "settings": {
+        "foreground": "#88C0D0"
+      }
+    },
+    {
+      "name": "[Markdown] Markup Math Operator",
+      "scope": "text.html.markdown punctuation.math.operator.latex",
+      "settings": {
+        "foreground": "#81A1C1"
+      }
+    },
     {
       "name": "[Markdown] Punctuation Definition Heading",
       "scope": "text.html.markdown punctuation.definition.heading",