Use pnpm instead of npm for frontend resources

[pleku]

As based on #6150, we will add support for pnpm for Vaadin 14.x / Flow 2.x and then bring it as

• The default mode for new v14 projects
• Keep existing projects working 14 projects working
• Add a turn-key way to go from a 14 npm project to a 14 pnpm project. This might not be initially available, but will be added at some point
• There will be only one package.json that is managed fully by the framework when you are using pnpm mode

Parts:

• Check that pnpm installed locally (not globally) works in the same way as globally installed #7019 Check that pnpm installed locally works the same way as globally installed
• Install pnpm after validating node and npm version #7020 Install pnpm after validating node and npm version
• Use pnpm instead of npm for frontend resources #7021 Use pnpm instead of npm for frontend resoures
• Document change from pnpm to npm as the default #7041 Document change from pnpm to npm as the default
• Switching between npm and pnpm seamlessly #7042 Switching between npm and pnpm seamlessly

[Legioth]

The part about using only one package.json may need some additional consideration to take into account hybrid use cases where some dependencies are managed "manually" (e.g. running npm install) and some are managed through @NpmPackage annotations on the classpath. The tricky part there is how to know what to do in situations when an @NpmPackage annotation is removed or if its version number is updated.

This is not so much of an issue for Vaadin 14.x where this kind of hybrid approach isn't really encouraged, but it will be problematic in Vaadin 15.

[manolo]

This use case should work out-of-the-box with whatever solution proposed:

• Developer annotates their java class with @NpmPackage(value="@vaadn/grid", version="latest") (notice the typo)
• An errored entry is added to the package.json failing npm install.

"@vaadin/vaadn-grid": "latest",

• The the user fixes the typo in java, then the wrong entry should be removed and the correct one added.

"@vaadin/vaadin-grid": "latest",

Otherwise, the project would never build, and the user needs to know what file and what to remove to fix the project. Note that mvn clean would not help here