Add Nim to default languages

Nim is a systems programming language (included in the highlight.js
`system` group), and we're quite happily using `mdBook` in several of
our documentation projects starting with our [style
guide](https://status-im.github.io/nim-style-guide/).

While we can maintain our own highlight.js, including `Nim` in the
default distribution would allow us to promote more mdBook usage in the
Nim community at the cost of a ~2kb increase in the `highlight.js` size.

CONTRIBUTING.md

@@ -148,7 +148,7 @@ The following are instructions for updating [highlight.js](https://highlightjs.o
 1. Clone the repository at <https://github.com/highlightjs/highlight.js>
 1. Check out a tagged release (like `10.1.1`).
 1. Run `npm install`
-1. Run `node tools/build.js :common apache armasm coffeescript d handlebars haskell http julia nginx properties r scala x86asm yaml`
+1. Run `node tools/build.js :common apache armasm coffeescript d handlebars haskell http julia nginx nim properties r scala x86asm yaml`
 1. Compare the language list that it spits out to the one in [`syntax-highlighting.md`](https://github.com/camelid/mdBook/blob/master/guide/src/format/theme/syntax-highlighting.md). If any are missing, add them to the list and rebuild (and update these docs). If any are added to the common set, add them to `syntax-highlighting.md`.
 1. Copy `build/highlight.min.js` to mdbook's directory [`highlight.js`](https://github.com/rust-lang/mdBook/blob/master/src/theme/highlight.js).
 1. Be sure to check the highlight.js [CHANGES](https://github.com/highlightjs/highlight.js/blob/main/CHANGES.md) for any breaking changes. Breaking changes that would affect users will need to wait until the next major release.

guide/src/format/theme/syntax-highlighting.md

@@ -44,6 +44,7 @@ your own `highlight.js` file:
 - makefile
 - markdown
 - nginx
+- nim
 - objectivec
 - perl
 - php

test_book/src/languages/README.md

@@ -27,6 +27,7 @@ This Currently contains following languages
 - makefile
 - markdown
 - nginx
+- nim
 - objectivec
 - perl
 - php

test_book/src/languages/highlight.md

@@ -529,6 +529,26 @@ http {
 }
 ```
 
+## nim
+
+```nim
+from strutils import `%`
+
+const numDoors = 100
+var doors {.compileTime.}: array[1..numDoors, bool]
+
+proc calcDoors(): string =
+  for pass in 1..numDoors:
+    for door in countup(pass, numDoors, pass):
+      doors[door] = not doors[door]
+  for door in 1..numDoors:
+    result.add("Door $1 is $2.\n" % [$door, if doors[door]: "open" else: "closed"])
+
+const outputString: string = calcDoors()
+
+echo outputString
+```
+
 ## objectivec
 
 ```objectivec