Simplify type checking for objects

[msujew]

What it does

A follow up on #11490 (cc @paul-marechal)

Introduces an Is namespace with helpful methods to identify types. Uses the newly introduced methods to simplify type checking for objects. I didn't use the Is.string, Is.number etc functions as that would make the PR even larger and I wasn't sure whether it would make sense to use the functions that way anyway.

For some reason, this change required an additional call to reflect-metadata in the plugin host. Otherwise it would crash as the ContributionFilterRegistryImpl would try to annotate itself without the Reflect.hasOwnMetadata function available.

How to test

Nothing should've changed

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

@msujew should we maybe update packages/core/src/common/types.ts to include similar methods present in vscode (copying them over)?

As trivial as the checks might be I think unit-testing them might be good too.

[paul-marechal]

I pushed a commit with an alternative writing: The reasoning is that the function names should better describe what it does (isObject vs object) and if users want to use the functions under a namespace then can import as:

import * as types from '@theia/core/lib/common/types';
// ...
types.isObject(...);

[paul-marechal]

I like these new functions as they greatly help when dealing with unknown-typed variables!

[paul-marechal]

@msujew feel free to merge if you agree with the changes suggested in the added commit, otherwise drop it before.

[msujew]

@paul-marechal Thanks, I didn't have time recently to update this PR :)

Your changes look good to me 👍