Module name mapper regex

[StenCalDabran]

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?

[x] Bugfix
[x] 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: #750

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

The first commit fixes the problem addressed in the issue. The second commit makes the generated moduleNameMapper entries more clear by only adding one entry per library.
The fixed original generated entry would be:

"moduleNameMapper": {
  "^@app/my-lib$": "<rootDir>/libs/my-lib/src",
  "^@app/my-lib/(.*)$": "<rootDir>/libs/my-lib/src/$1",
}

The new generated entry instead is:

"moduleNameMapper": {
  "^@app/my-lib(|/.*)$": "<rootDir>/libs/my-lib/src/$1",
}

It matches either an empty string or everything starting with a slash to be inserted into $1.
The trailing slash should present no problem.
However, I would understand if you only want to merge the fix commit, in that case I can change the PR.

[kamilmysliwiec]

LGTM, thanks!