feat() support project selection in add action

[johnbiundo]

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
[x] Feature
[x] Code style update (formatting, local variables)
[x] 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?

See below

Does this PR introduce a breaking change?

[ ] Yes
[x] No

But, see below

Other information

Main feature

This PR introduces a new feature for the nest add command. The command now:

• Allows passing in a project name with -p option
• Automatically handles multi-project selection if a project is not supplied
  • checks for whether the project structure is standard or monorepo
  • if monorepo, prompts for the project name
• Invokes the nest-add schematic on the target library and passes in the source root via --sourceRoot

Minor changes

• improve help messages (slight wording/grammar improvements; consistent title casing and periods, as guided by the Angular CLI conventions)
• improve test script: run npm clean before running tests
• refactor utils for monorepo/project selection into library functions
• refactor generate action/command to use above utils
• add display of CLI version to info command
• update readme to be consistent with docs

Compatibility

• Monorepos introduced an incompatibility with any libraries that implemented a nest-add schematic because the schematic would be unaware of a monorepo structure.
• It's anticipated that there are very few, if any, such 3rd party libraries in the wild. Certainly, I'm not aware of any bug reports from such authors after having introduced monorepos, though it may be early.
• There is at least one @nestjs library that implements the nest-add schematic: https://github.com/nestjs/azure-func-http. @kamilmysliwiec may know of more.
• In order to function properly in a monorepo, with this supporting change in place, library authors implementing a nest-add schematic should accept a --sourceRoot flag and install code relative to the supplied path.

[kamilmysliwiec]

There are 2 officially supported packages that implement nest-add schematic:

• https://github.com/nestjs/azure-func-http
• https://github.com/nestjs/ng-universal

Would you like to create PRs for them? (in order to adjust to the current API)

[kamilmysliwiec]

Let's track this separately:

• Support "nest add" in monorepo ng-universal#199
• Support "nest add" in monorepo azure-func-http#93