feat: graph inspector, preview mode

[kamilmysliwiec]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build f4120a86-4728-4202-bd5e-83e87d442ec0

• 245 of 288 (85.07%) changed or added relevant lines in 20 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 93.376%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/core/injector/modules-container.ts	0	1	0.0%
packages/core/inspector/graph-inspector.ts	47	48	97.92%
packages/core/inspector/noop-graph-inspector.ts	3	4	75.0%
packages/core/router/paths-explorer.ts	16	17	94.12%
packages/websockets/web-sockets-controller.ts	7	8	87.5%
packages/core/injector/injector.ts	2	4	50.0%
packages/core/inspector/serialized-graph.ts	37	43	86.05%
packages/core/injector/module.ts	14	23	60.87%
packages/microservices/listeners-controller.ts	14	23	60.87%
packages/core/scanner.ts	32	44	72.73%

Totals
Change from base Build 17232e0f-baa2-44ac-93c8-f2ee042ba748:	-0.03%
Covered Lines:	6358
Relevant Lines:	6809

---

💛 - Coveralls