Adds support for #[wasm_bindgen(typescript_custom_section)].

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

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+# `typescript_custom_section`
+
+When added to a `const` `&'static str`, it will append the contents of the
+string to the `.d.ts` file exported by `wasm-bindgen-cli` (when the
+`--typescript` flag is enabled).
+
+```rust
+#[wasm_bindgen(typescript_custom_section)]
+const TS_APPEND_CONTENT: &'static str = r#"
+
+export type Coords = { "latitude": number, "longitude": number, }; 
+
+"#;
+```
+
+The primary target for this feature is for code generation. For example, you
+can author a macro that allows you to export a TypeScript definition alongside
+the definition of a struct or Rust type.
+
+```rust
+#[derive(MyTypescriptExport)]
+struct Coords {
+    latitude: u32,
+    longitude: u32,
+}
+```
+
+The proc_derive_macro "MyTypescriptExport" can export its own
+`#[wasm_bindgen(typescript_custom_section)]` section, which would then be
+picked up by wasm-bindgen-cli. This would be equivalent to the contents of
+the TS_APPEND_CONTENT string in the first example.
+
+This feature allows plain data objects to be typechecked in Rust and in
+TypeScript by outputing a type definition generated at compile time.