Restructure project directories (#333)

`/client/web` -> `/frontend`
`/client/cli` -> *delete for now*
`/client/native` -> *delete for now*
`/core/editor` -> `/editor`
`/core/document` -> `/graphene`
`/core/renderer` -> `/charcoal`
`/core/proc-macro` -> `/proc-macros` *(now plural)*

frontend/assets/16px-solid/flip-horizontal.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">
+	<path d="M15,4.6v6.8l-4-2V6.6L15,4.6 M16,3l-6,3v4l6,3V3L16,3z" />
+	<polygon points="5,10 0,13 0,3 5,6" />
+	<rect x="7" y="13" width="1" height="2" />
+	<rect x="7" y="9" width="1" height="2" />
+	<rect x="7" y="5" width="1" height="2" />
+	<rect x="7" y="1" width="1" height="2" />
+</svg>