Add link-brewed-php to help with PHP versioning

Basically when you install multiple PHPs with Homebrew, they don't end
up on PATH anywhere - you can brew link one of them, but that overwrites
any other PHPs you might have previously linked.

Other platforms, such as Ubuntu, package their PHPs such that a binary
called something like php7.4 ends up on PATH. This is a much more
helpful approach, since you can specifically invoke the necessary
version of PHP by adding a version suffix.

This script does the same thing for Homebrew-installed PHPs. Pretty
simple.

local/bin/link-brewed-php

@@ -0,0 +1,7 @@
+#!/usr/bin/env zsh
+cd ~/bin || exit $?
+for bin in /usr/local/opt/php@*/bin/php; do
+	name=php${${bin##*@}%%/*}
+	echo "Linking ~/bin/$name -> $bin" >&2
+	ln -fs $bin $name
+done