Add skip_typescript attribute to prevent .d.ts emit (#2016)

* Add skip_typescript attribute to prevent .d.ts emit

* Add guide page for typescript attribute

guide/src/reference/attributes/on-rust-exports/skip_typescript.md

@@ -0,0 +1,42 @@
+# `skip_typescript`
+
+By default, Rust exports exposed to JavaScript will generate TypeScript definitions (unless `--no-typescript` is used). The `skip_typescript` attribute can be used to disable type generation per function, enum, struct, or field. For example:
+
+```rust
+#[wasm_bindgen(skip_typescript)]
+pub enum MyHiddenEnum {
+    One,
+    Two,
+    Three
+}
+
+#[wasm_bindgen]
+pub struct MyPoint {
+    pub x: u32,
+
+    #[wasm_bindgen(skip_typescript)]
+    pub y: u32,
+}
+
+#[wasm_bindgen]
+impl MyPoint {
+
+    #[wasm_bindgen(skip_typescript)]
+    pub fn stringify(&self) -> String {
+        format!("({}, {})", self.x, self.y)
+    }
+}
+```
+
+Will generate the following `.d.ts` file:
+
+```ts
+/* tslint:disable */
+/* eslint-disable */
+export class MyPoint {
+  free(): void;
+  x: number;
+}
+```
+
+When combined with [the `typescript_custom_section` attribute](typescript_custom_section.html), this can be used to manually specify more specific function types instead of using the generated definitions.