Add missing code font var

Code blocks use a different variable, which this commit adds.

On the current Nushell website, for example [Quick Tour][quicktour] page,
we can see that the font-family being applied is in the generated style-*.css,
with declaration `code { font-family(--code-font-family); }`,
which does not match our supposed override which defines a var
under a different name `--font-family-code`.

This var name has been used since the introduction of the override in 83ced1d.

Maybe vuepress changed the variable at some point?

This is a follow-up to 2ec6143 (PR nushell#1890, PR nushell#1906),
which attempts to solve Android monospace due to missing font coverage (nushell#83).

Instead of replacing `--font-family-code`, `--code-font-family` is added as an additional value-identical declaration just in case the other variable is being used somewhere still.

[quicktour]: https://www.nushell.sh/book/quick_tour.html#nushell-commands-output-data

Author: Kissaki

.vuepress/styles/index.scss

@@ -38,6 +38,8 @@ div[class*='language-'].line-numbers-mode .line-numbers {
   --code-line-height: 1.1;
   --font-family-code: 'Fira Code', Consolas, Monaco, 'Andale Mono', 'DejaVu Sans Mono',
     'Ubuntu Mono', monospace;
+  --code-font-family: 'Fira Code', Consolas, Monaco, 'Andale Mono', 'DejaVu Sans Mono',
+    'Ubuntu Mono', monospace;
 }
 
 @media screen and (min-width: 720px) and (max-width: 815px) {