Adding [Symbol.toStringTag] to Bluebird

[dancrumb]

This is a rehash of #10831, which was closed because Bluebird did not have this Symbol available back then.

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If changing an existing definition:

• Follows on from Add Symbol.toStringTag to promise instance petkaantonov/bluebird#1277

[typescript-bot]

@dancrumb Thank you for submitting this PR!

🔔 @lhecker - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[lhecker]

Unfortunately this PR is a bit incomplete and has a number of issues (I'm sorry):

• Test cases for this new functionality are missing
• The Symbol.toStringTag field on Bluebird promises actually returns "Object" instead of "Promise"
• The actual signature of the property is readonly - see here
• This in turn requires a TypeScript version upgrade of Bluebird's typings and all of its transitive dependents
• Bluebird wasn't actually released yet with this feature unfortunately - see here

I humbly suggest to close this PR in favor of #34805.

[dancrumb]

Gladly!