Address version conflict when used together with firebase-admin

[Feiyang1]

This PR will allow firebase-admin to remove @firebase/app from its dependency list. @firebase/app being a dependency in firebase-admin causes version conflict when firebase-admin and firebase are used together. More detail: #1696 (comment)

The PR includes the following changes

1. Introduce a new file version.ts in @firebase/database, so the source code doesn't import @firebase/app directly for SDK_VERSION.
2. set SDK_VERSION in version.ts in the entry files (index.ts and index.node.ts), so we can set SDK_VERSION differently depending on the target.
3. change the static import firebase from '@firebase/app' to require('@firebase/app').default in index.node.ts for handling firebase-admin use case. It allows @firebase/database to work in firebase-admin without @firebase/app

The corresponding PR in firebase-admin: firebase/firebase-admin-node#586

[hsubox76]

Could this swallow a legit error in registerDatabase()? If the require works fine but there is some error thrown in registerDatabase() itself?

[Feiyang1]

Yeah, updated to only ignore MODULE_NOT_FOUND error

[schmidt-sebastian]

I believe this export in combination with setSDKVersion doesn't work, as it exports the current state of the variable:

deps.js:

let foo = 'foo'
function setToBar() {
	foo = 'bar';
}
module.exports = {foo, setToBar};

let deps = require('./deps');
console.log(deps.foo);
deps.setToBar();
console.log(deps.foo);

Prints:

foo
foo

[Feiyang1]

It actually works because esm only exports references. The code compiled to cjs format would look like:

exports.foo = 'foo';
function setToBar() {
    exports.foo = 'bar';
}
exports.setToBar = setToBar;

[Feiyang1]

@schmidt-sebastian gentle ping :)

[schmidt-sebastian]

LGMT based on your ESM research