chore: add files field in package.json to only include dist files

[ydcjeff]

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: package distribution

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

By only including distribution files, this could reduce the package size.

[kamilmysliwiec]

What's the reason for this change?

[ydcjeff]

With this, users don't need to download unnecessary files to use the package. Currently, the distribution includes test directory, coverage, tsconfig files, etc which are not needed from the user view point.

I noticed other @nestjs/* packages that I used has fields defined, but this one doesn't.

[ydcjeff]

Can we merge this PR? @kamilmysliwiec

[kamilmysliwiec]

lgtm