improve package.json parsing performance

[jrieken]

I have made the below modification to the ExtensionManifestParser and it seems that the general performance of our JSON in comparing to the native JSONparse is OK'ish there are cases in which it is very bad. For instance parsing css/package.json takes 0.3ms using JSON.parse it takes 8ms using our parser. Other larger json files show similar behaviour.

let t1 = process.hrtime();
const extensionDescription = json.parse(manifestContents.toString(), errors);
let d = process.hrtime(t1);
let D1 = (d[0] * 1e9 + d[1]) / 1e6;

let t2 = process.hrtime();
JSON.parse(manifestContents.toString());
let d2 = process.hrtime(t2);
let D2 = (d2[0] * 1e9 + d2[1]) / 1e6;

console.log(`PARSE + ${this._absoluteManifestPath} took ${D1}ms, NATIVE ${D2}ms`);

[jrieken]

Assigning @aeschli to see if there is something low hanging that can be improved and @sandy081 to check if we can get away with stripping comments and using JSON.parse

[aeschli]

Comments are not valid in package.json. We can use the native json.parser. The only difference is that our implementation is fault tolerant, but IMO that's not important here.
I had a quick look at our parser implementation and also improved it slightly, but it's unlikely that we can match the native implementation.