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/12px-solid/fullscreen-exit.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 12 12">
+	<polygon points="9,12 9,9 12,9 12,8 8,8 8,12" />
+	<polygon points="3,3 0,3 0,4 4,4 4,0 3,0" />
+	<polygon points="0,9 3,9 3,12 4,12 4,8 0,8" />
+	<polygon points="9,3 9,0 8,0 8,4 12,4 12,3" />
+</svg>