asdf: add Homebrew and completion support (#6749)

* Modifies the search logic for asdf to include Homebrew when it is
installed. The implementation is adapted from the pyenv plugin.

plugins/asdf/asdf.plugin.zsh

-# Find where asdf should be installed.
+# Find where asdf should be installed
 ASDF_DIR="${ASDF_DIR:-$HOME/.asdf}"
 
-# Load asdf, if found.
-if [ -f $ASDF_DIR/asdf.sh ]; then
-    . $ASDF_DIR/asdf.sh
+# If not found, check for Homebrew package
+if [[ ! -d $ASDF_DIR ]] && (( $+commands[brew] )); then
+   ASDF_DIR="$(brew --prefix asdf)"
 fi
 
-# Load asdf completions, if found.
-if [ -f $ASDF_DIR/completions/asdf.bash ]; then
-    . $ASDF_DIR/completions/asdf.bash
+# Load command
+if [[ -f "$ASDF_DIR/asdf.sh" ]]; then
+    . "$ASDF_DIR/asdf.sh"
+
+    # Load completions
+    if [[ -f "$ASDF_DIR/completions/asdf.bash" ]]; then
+        . "$ASDF_DIR/completions/asdf.bash"
+    fi
 fi