Document webpack change: refactor filesystem interfaces

[webpack-bot]

A pull request by @sokra was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#9251

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add Compiler.intermediateFileSystem
avoid using path and fs module when possible
move join, mkdirp and dirname into utils
join and dirname is optional in FileSystem interface
remove mkdirp from Filesystem interface

What kind of change does this PR introduce?
refactoring

Did you add tests for your changes?
existing tests

Does this PR introduce a breaking change?
yes

What needs to be documented once your changes are merged?

• new property intermediateFileSystem on the Compiler
  • allows to override the file system for all actions by plugins that are not considered as output
  • Plugins should use this file system to write and read intermediate files
  • It defaults to fs.
• mkdirp is no longer expected to be a function on the output file system
• join is now optional on the output file system
• dirname is a new optional method on the output file system
• the output filesystem defaults to fs now
• NodeOutputFileSystem class has been removed