feat: expose dist paths as root paths via package.json export field

[pskelin]

Overview

Enable writing imports like
import "@ui5/webcomponents/Button.js"
instead of
import "@ui5/webcomponents/dist/Button.js"

The dist folder is a development artefact and is not relevant for application developers, so this becomes a more developer friendly way of consuming ui5 webcomponents.

This feature relies on the package.json exports field and should be considered only if it is supported. The original import paths with dist remain available and are the only official imports. Users of this features must take care that their build tool has the necessary support and that multiple copies of the same module do not end up in the final bundle in case they are referenced by both import paths and the build tool does not handle this correctly (checked to work correctly with rollup and webpack at this time)

if for example app developers write imports like

import "@ui5/webcomponents/Button.js"
import "@ui5/webcomponents/ToggleButton.js"

since ToggleButton internally imports
import "@ui5/webcomponents/dist/Button.js"

there should be just one copy of the Button module even if it is imported by both paths.

Remarks

Other files that are not actual modules but are used by tools via require.resolve to get the correct monorepo folder need to also be exposed in the exports field (examples are the .hbs files in src for template includes, the .port file used by the tests and the bundle files used by rollup)

resolves #3208
alternative considered in #3292