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/wasm/src/shims.rs

@@ -0,0 +1,10 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+extern "C" {
+	#[derive(Clone, Debug)]
+	pub type Error;
+
+	#[wasm_bindgen(constructor)]
+	pub fn new(msg: &str) -> Error;
+}