Signed-off-by: Reece Dunham me@rdil.rocks

Motivation

Right now, @docusaurus/core is 1.36mb, which seems a bit big for just the core files. This reduces the size of a number of the packages by doing a number of things:

• Updates the TypeScript target to ES2019 from ES2017, since Node 12 is now the required minimum for D2. This means that some modern syntax features don't need to be polyfilled.
• Changes the TypeScript compiler settings to avoid writing out helper functions like this just to use the spread operator:

__rest = function (s, e) {
    var t = {};
    for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p) && e.indexOf(p) < 0)
        t[p] = s[p];
    if (s != null && typeof Object.getOwnPropertySymbols === "function") {
        for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++) {
            if (e.indexOf(p[i]) < 0 && Object.prototype.propertyIsEnumerable.call(s, p[i]))
                t[p[i]] = s[p[i]];
        }
    }
    return t;
};

To instead just import it from tslib, reducing code duplication.

• Disabling the generation of source maps from .d.ts files, which is just unneeded since the .d.ts are generated in the first place. They are essentially their own source maps.
• Update TypeScript to v4.1.5

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Everything appears to be fine.

Related PRs

n/a