Install pnpm after validating node and npm version

[mehdi-vaadin]

Part of #6966 Depends on #7019

With setting a configuration key, e.g. disable.pnpm, the user should be able to switch to npm; in that case, pnpm doesn't need to be installed.

[joheriks]

Acceptance criteria:

• After implementation, (a fixed version of) pnpm is automatically installed as a local module unless disabled
• The parameter disable.pnpm should be available both as a Maven parameter and a Vaadin configuration property
• Document the new property in https://vaadin.com/docs/v14/flow/advanced/tutorial-all-vaadin-properties.html and https://vaadin.com/docs/v14/flow/production/tutorial-production-mode-advanced.html

[denis-anisimov]

This ticket will be in fact superseded by the #7021 Validation which is mentioned in the description is a bad place to install pnpm for many reasons:

• validation in the plugin is done inside prepare-frontend goal. This is a bad place to install pnpm since the goal is used in both dev and production mode. If you are using dev mode then you will need to disable pnpm twice: in the prepare-frontend via config parameter and in the runtime via deployment config parameter. The problem is : maven config parameter can't contain dots and it never matches the servlet init parameter. You may use a system property for both cases. But system property for maven plugin will be disable.pnpm and vaadin.disable.pnpm for servlet (and it's disable.pnpm as an init parameter property). As a result it's strongly inconvenient to use two properties to disable one feature. We may decide later on to install pnpm in prepare-frontend for the sake of speed but this should be a totally separate consideration and a ticket.
• It's quite artificial to install pnpm without a need: pnpm should be installed at the first time when it's needed. And since this ticket is not about pnpm usage it can't be installed without usage. Again: later on we may decide to install it in advance to speed up things. But the above consideration should be taken into account. This should be a separate task.