build: speed up the TypeScript transpilation when using webpack

[tinogo]

Disable the type-checking while transpiling the TS files, as the ForkTsCheckerWebpackPlugin
is already handling this within another process

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)
[x] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Run nest build for a webpack based Nest.js-CLI project (it should be considerably big).

Issue Number: N/A

What is the new behavior?

With ts-loaders transpileOnly-flag set to true, the transpilation process gets a serious speed up, as it disables the type-checking while transpiling the sources.
It is still safe to do this, as the type checking takes place in another process using the ForkTsCheckerWebpackPlugin.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[tinogo]

Just to give you some numbers:
For our project at work, this change halves the time at takes to bundle the app.
The initial compilation drops from ca. 35 sec. to 17-18 sec. :)

[kamilmysliwiec]

Thank you @tinogo!

[tblue1994]

Looks like this change breaks the nestjs swagger plugin's ability to automatically resolve circular dependencies. When we upgraded to 7.2.0, we started getting the following error:

UnhandledPromiseRejectionWarning: Error: A circular dependency has been detected (property key: "statuses"). Please, make sure that each side of a bidirectional relationships are using lazy resolvers ("type: () => ClassType").

However, this was working in 7.1.5, and if I manually turn transpileOnly off, the app will start.

[kamilmysliwiec]

@tblue1994 can you please create a separate issue in @nestjs/swagger with a minimum reproduction repo?