feat: standardized handling `$0`

For all kinds of plugin managers to get the plugin’s location properly, we need to handle `$0` correctly.

zsh-autosuggestions.plugin.zsh

@@ -1 +1,6 @@
-source ${0:A:h}/zsh-autosuggestions.zsh
+# According to the standard:
+# https://github.com/zdharma/Zsh-100-Commits-Club/blob/master/Zsh-Plugin-Standard.adoc
+0="${ZERO:-${${0:#$ZSH_ARGZERO}:-${(%):-%N}}}"
+0="${${(M)0:#/*}:-$PWD/$0}"
+
+source ${0:h}/zsh-autosuggestions.zsh