feat(core): allow a component to register nesting components

[raymondfeng]

In a large project, a component can have dependencies on other components. This feature makes it easier to mount required components to the application.

Checklist

• [x] DCO (Developer Certificate of Origin) signed in all commits
• [x] npm test passes on your machine
• [x] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide
• [ ] API Documentation in code was updated
• [ ] Documentation in /docs/site was updated
• [ ] Affected artifact templates in packages/cli were updated
• [ ] Affected example projects in examples/* were updated

👉 Check out how to submit a PR 👈

[coveralls]

Pull Request Test Coverage Report for Build 9419540479

Details

• 3 of 7 (42.86%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 55.287%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/core/src/application.ts	2	3	66.67%
packages/core/src/component.ts	1	4	25.0%
<!--	Total:	3	7	42.86%	-->

Totals
Change from base Build 9409318021:	-0.02%
Covered Lines:	9566
Relevant Lines:	12465

---

💛 - Coveralls