Implements JavaScript constraints

[arcanis]

What's the problem this PR addresses?

Using Prolog for constraints was an interesting experiment, and I don't regret it, but the developer experience is ultimately too poor for wide adoption. Given how useful are constraints otherwise, I want to switch the engine to a JS-powered one, while keeping compatibility with the Prolog one for at least one major.

Fixes #1276

How did you fix it?

With this diff, users can add a yarn.config.js file at the root of their repository. If it exists, yarn constraints will evaluate it, retrieve its constraints export, and run it. The function can perform a couple of "offline mutations" which are applied after all the validation has run (if --fix is set; otherwise, it just exits with an exit code).

The output was also updated to a more modern display, with an extra nicety for JS constraints: conflicts show the exact location where the mutation calls have been made, making it easier to debug conflicting rules.

Here's what part of our Prolog file looks like when implemented in JS:

exports.constraints = Yarn => {
    // This rule will enforce that a workspace MUST depend on the same version of
    // a dependency as the one used by the other workspaces
    for (const u of Yarn.dependencies())
        for (const v of Yarn.dependencies({ident: u.ident}))
            if (u.type !== `peerDependencies` && v.type !== `peerDependencies`)
                if ((u.type !== `devDependencies` && v.type !== `devDependencies`) || !Yarn.workspace({ident: u.ident}))
                    u.update(v.range);

    // This rule will prevent workspaces from depending on non-workspace versions
    // of available workspaces
    for (const u of Yarn.dependencies({}))
        if (Yarn.workspace({ident: u.ident}))
            u.update(`workspace:^`);

    // This rule enforces that all packages must not depend on inquirer
    for (const u of Yarn.dependencies({ident: `inquirer`}))
        u.error(`We use enquirer rather than inquirer`);

    // This rule enforces specific fields on all our workspaces
    for (const w of Yarn.workspaces()) {
        w.set(`repository.type`, `git`);
        w.set(`repository.url`, `ssh://[email protected]/yarnpkg/berry.git`);
        w.set(`repository.directory`, w.cwd);
    }
};

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.