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/24px-two-tone/vector-line-tool.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24">
+	<path style="fill:#65BBE5;" d="M5.83,21.71h1.66c0.75,0,1.47-0.3,2-0.83l11.4-11.4c0.53-0.53,0.83-1.25,0.83-2V5.83L5.83,21.71z" />
+	<polygon style="fill:#FFFFFF;" points="20.29,2.29 2.29,20.29 3.71,21.71 21.71,3.71" />
+</svg>