feat(core): enhance repl '.help' command with nestjs functions

[micalevisk]

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?

when using the REPL, typing .help only displays nodejs REPL commands

What is the new behavior?

People that are used with Node.js REPL are aware of the .help command and could try to use it in their Nest REPL. As a DX improvement, help() native function's output could be shown when using .help alongside with all available commands. Like this:

Another improvement on the .help command: if the user supplies an arg to it (eg: .help save or .hepl debug), the help message will be shown instead.

Now we got several ways to view helper messages on nestjs REPL.

Does this PR introduce a breaking change?

• Yes
• No

[coveralls]

Pull Request Test Coverage Report for Build 05b66a2e-5791-42d8-b42a-d6f7ff175a3e

• 2 of 19 (10.53%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 93.167%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/core/repl/repl.ts	1	2	50.0%
packages/core/repl/repl-native-commands.ts	1	17	5.88%

Files with Coverage Reduction	New Missed Lines	%
packages/core/repl/repl.ts	1	50.0%

Totals
Change from base Build a213a525-4c5b-48ee-9803-b76e72178509:	-0.2%
Covered Lines:	6204
Relevant Lines:	6659

---

💛 - Coveralls

[kamilmysliwiec]

lgtm