This commit reverts part of the implementation of [RFC 6]. That RFC
specified that the `--browser` flag was going to be repurposed for the
new "natively loadable as ES module output", but unfortunately the
breakage is far broader than initially expected. It turns out that
`wasm-pack` passes `--browser` by default which means that a change to
break `--browser` would break all historical versions of `wasm-pack`
which is a bit much for now.
To solve this the `--browser` flag is going back to what it represents
on the current released version of `wasm-bindgen` (optimize away some
node.js checks in a few places for bundler-style output) and a new
`--web` flag is being introduced as the new deployment strategy.
[RFC 6]: https://github.com/rustwasm/rfcs/pull/6Closes#1318
This commit is an implementation of [RFC 6] which enables crates to
inline local JS snippets into the final output artifact of
`wasm-bindgen`. This is accompanied with a few minor breaking changes
which are intended to be relatively minor in practice:
* The `module` attribute disallows paths starting with `./` and `../`.
It requires paths starting with `/` to actually exist on the filesystem.
* The `--browser` flag no longer emits bundler-compatible code, but
rather emits an ES module that can be natively loaded into a browser.
Otherwise be sure to check out [the RFC][RFC 6] for more details, and
otherwise this should implement at least the MVP version of the RFC!
Notably at this time JS snippets with `--nodejs` or `--no-modules` are
not supported and will unconditionally generate an error.
[RFC 6]: https://github.com/rustwasm/rfcs/pull/6Closes#1311
