Add Detect If You Are On A Mac as a vim til

vim/detect-if-you-are-on-a-mac.md

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+# Detect If You Are On A Mac
+
+There are a couple ways of detecting with vimscript if you are on a mac.
+This can be useful if you are writing a plugin with OS-specific
+functionality. Here are two ways to make that check.
+
+```vimscript
+if has('macunix') || has('mac') || has('osx')
+  ...
+endif
+```
+
+Alternatively, you can use Vim's `system()` function to execute unix's
+`uname` command. This command will give you the name of the operating
+system. In the event you are using a Mac, the result of `uname` should be
+`Darwin`. The following regex match is a good way to make this check.
+
+```vimscript
+if system('uname') =~ "Darwin"
+  ...
+endif
+```
+
+See `:h has()`, `:h system()`, and `man uname` for more details.