In bundler mode, generate the package.json with "type": "module" and use the
"main" attribute instead of the "module" attribute.

This change makes the built ES module palatable to node in
--experimental-wasm-modules mode as well as to webpack as illustrated in
webpack/webpack#14313

I noticed that this specific evasion of the "type" keyword is already used in
utils::manifest::NpmPackage in the tests, so just match that.

…n-package-json

Update test case that landed after this PR was created, in
rustwasm#1061, to expect main attribute in
package.json instead of module attribute.