Add Check For An Executable as a vim til

vim/check-for-an-executable.md

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+# Check For An Executable
+
+Sometimes Vim needs to reach outside of itself to use an existing program.
+For example, wiring up [auto-formatting of JavaScript
+code](https://hashrocket.com/blog/posts/writing-prettier-javascript-in-vim)
+requires Vim to call out to the
+[`prettier`](https://github.com/prettier/prettier) binary.
+
+We want our `.vimrc` files and plugins to, generally, be as portable as
+possible. What happens if you haven't yet installed a particular program?
+Vim will likely experience a runtime exception. One way to get around this
+is to check for the presence of that program on the path. If it isn't there,
+don't do the thing. We can use the `executable()` function for this.
+
+```vimscript
+if executable('prettier')
+  ...
+endif
+```
+
+It will return `1` (true) if `prettier` is an executable on the path,
+otherwise it will return `0` (false).
+
+See `:help executable()` for more details.