Arcanist plugin (#5106)

* Add initial list of aliases

* Add lint and diff aliases

* Remove usage section

plugins/arcanist/README.md

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+## arcanist
+
+**Maintainer:** [@emzar](https://github.com/emzar)
+
+This plugin adds many useful aliases.

plugins/arcanist/arcanist.plugin.zsh

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+#
+# Aliases
+# (sorted alphabetically)
+#
+
+alias ara='arc amend'
+alias arb='arc branch'
+alias arc='arc cover'
+alias arci='arc commit'
+
+alias ard='arc diff'
+alias ardnu='arc diff --nounit'
+alias ardnupc='arc diff --nounit --plan-changes'
+alias ardpc='arc diff --plan-changes'
+
+alias are='arc export'
+alias arh='arc help'
+alias arl='arc land'
+alias arli='arc lint'
+alias arls='arc list'
+alias arp='arc patch'