Make JS use a '.wasm' extension when importing the binary

[jamen]

Relevant change is

-                format!("import * as wasm from './{}_bg';", module_name)
+                format!("import * as wasm from './{}_bg.wasm';", module_name)

So the JS output becomes:

import * as wasm from './foo_bg.wasm';

This works the same on Webpack and lets you use rollup and rollup-plugin-wasm. 🎉

There is an argument to be explicit about the extensions in module resolution, but this was mostly about the Rollup for me, so the commonjs output and the tests have the same tendency to not use .js extensions.

Other changes are auto removal of trailing whitespace.

[fitzgen]

Neat -- thanks @jamen !