fix(util): handle more edge cases in looseEqual() util and add spec

[jacobmllr95]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

This PR makes the looseEqual() util more bulletproof and adds an extra spec for it.

The previous implementation failed at following tests:

Expected: false
Received: true

  65 |     // Files with slightly different dates
> 66 |     expect(looseEqual(file1, file3)).toBe(false)

Expected: false
Received: true

  102 |     // Different regex
> 103 |     expect(looseEqual(rx1, rx3)).toBe(false)

Expected: false
Received: true

> 153 |     expect(looseEqual(obj1, date1)).toBe(false)

Performance should also be equal or slightly better because early exits are used wherever possible:
https://jsperf.com/looseequal-comparison

Shoutout to @tmorehouse for helping with the tests and handling all the edge cases.

[jacobmllr95]

@posva @yyx990803 Since the same changes landed in Vue v3 it would be awesome to have them in Vue v2.7 too.

Ref: vuejs/core#379