zsh: Only source *.zsh files

Only source `*.zsh` files to prevent confusion or errors on startup.

Rename all files in 90-external-plugins accordingly.

.config/zsh/zshrc.d/90-external-plugins/20-autosuggestions.config.zsh

@@ -0,0 +1,13 @@
+# Set suggestions strategy
+# match_prev_cmd:
+#     Chooses the most recent match from history whose preceding item matches
+#     the most recently executed command.
+#     Won't work as expected with HIST_IGNORE_ALL_DUPS or HIST_EXPIRE_DUPS_FIRST
+# completion:
+#     Chooses a suggestion based on what tab-completion would suggest.
+ZSH_AUTOSUGGEST_STRATEGY=(match_prev_cmd completion)
+
+ZSH_AUTOSUGGEST_MATCH_NUM_PREV_CMDS=2
+ZSH_AUTOSUGGEST_MATCH_PREV_MAX_CMDS=-1
+
+# vim: ft=zsh