fix(output-file-system): change from backslash to forward slash in join

[soilSpoon]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

In webpack, if there is a join method of outputFileSystem, use the join method. However, if you use the join method in windows os, a problem occurs when the path is relative.
Ex)
before join:
/app/js
after join:
\app\join

If it changes as above, the following error occurs.

D:\foo\node_modules\webpack\lib\util\fs.js:141
                throw new Error(
                ^

Error: \js\app.js is neither a posix nor a windows path, and there is no 'dirname' method defined in the file system
    at dirname (D:\foo\node_modules\webpack\lib\util\fs.js:141:9)
    at D:\foo\node_modules\webpack\lib\Compiler.js:790:19
    at arrayIterator (D:\foo\node_modules\neo-async\async.js:3467:9)
    at timesSync (D:\foo\node_modules\neo-async\async.js:2297:7)
    at Object.eachLimit (D:\foo\node_modules\neo-async\async.js:3463:5)
    at emitFiles (D:\foo\node_modules\webpack\lib\Compiler.js:536:13)
    at D:\foo\node_modules\webpack\lib\util\fs.js:182:5
    at Immediate.<anonymous> (D:\foo\node_modules\memfs\lib\volume.js:684:17)
    at processImmediate (internal/timers.js:461:21)

This PR solves that problem.

Breaking Changes

The join method of outputFileSystem returns all backslashes replaced with slashes.

Additional Info

https://github.com/webpack/webpack/blob/master/lib/util/fs.js#L133-L145

[codecov]

Codecov Report

Merging #804 (093b4d5) into master (7a888f8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #804   +/-   ##
=======================================
  Coverage   99.22%   99.23%           
=======================================
  Files          10       10           
  Lines         259      261    +2     
  Branches       83       83           
=======================================
+ Hits          257      259    +2     
  Misses          2        2           

Impacted Files	Coverage Δ
src/utils/setupOutputFileSystem.js	100.00% <100.00%> (ø)

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[alexander-akait]

Please provide a problem

[alexander-akait]

Also this code for webpack@4

[alexander-akait]

Using regexp to change \ on / is bad idea

[soilSpoon]

Suggested change

	newPath.join = (...files) => path.join(...files).replace(/[\\/]+/g, '/');
	newPath.join = (...files) => path.join(...files).replaceAll('\\', '/');

Would this be better?

[soilSpoon]

Please provide a problem

@alexander-akait Can you please let me know what more explanations are needed?

[alexander-akait]

Please show me a problem why you need to change it

[alexander-akait]

Simple reproducible test repo

[soilSpoon]

@alexander-akait Sorry i think i was mistaken