add experiments.layers

[sokra]

• add experiments.layers
• add layers to define different module layers
• support module layers in stats
• add layer support for NormalModules

fixes #11466

What kind of change does this PR introduce?
feature

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?

• experiments.layers: true enables the experimental feature
• entry.xxx.layer: "name-of-layer" allows to set the layer for an entrypoint
• module.rules[].issuerLayer: RuleCondition allows to filter/match by layer of issuer
• module.rules[].layer: "name-of-layer" allows to set the layer of an module
• splitChunks.cacheGroups.xxx.layer allows to select modules by layer
• externals.byLayer["name-of-layer"] allows to specify externals by layer
• externals function has an additional argument property contextInfo which contains info about the issuer (e. g. the layer)

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

I've created an issue to document this in webpack/webpack.js.org.

[ndelangen]

I'd be great to get a documentation page explaining how to properly use this, meaning not from an API stand-point, but from an architecture one.

for example here: https://webpack.js.org/concepts/