fix types: accept a readonly array for configuration options

[pcorpet]

The webpack main function can get multiple configuration as an array. However this array doesn't need to be mutable. This fix makes it accept readonly arrays.

What kind of change does this PR introduce?

bugfix for typescript types declaration

Did you add tests for your changes?

No: I couldn't find typing tests.

Does this PR introduce a breaking change?

No: all types that were accepted before are still accepted, it accepts new types.

What needs to be documented once your changes are merged?

Nothing, this is a fix.

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This change is 

[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)

[sokra]

This is a generated file. Look into lib/webpack.js or lib/index.js and fix it in the jsdocs comments

[pcorpet]

This is a generated file. Look into lib/webpack.js or lib/index.js and fix it in the jsdocs comments

Thanks @sokra , I hadn't noticed.

I'm now stuck on microsoft/TypeScript#42768

			if (Array.isArray(options)) {
				/** @type {MultiCompiler} */
				compiler = createMultiCompiler(options, options);
				watch = options.some(options => options.watch);
				watchOptions = options.map(options => options.watchOptions || {});
			} else {
				/** @type {Compiler} */
				compiler = createCompiler(options);
				watch = options.watch;
				watchOptions = options.watchOptions || {};
			}

The isArray only guards propery against mutable arrays, not readonly arrays (see the bug in Typescript for deeper details).
Maybe we could cast the type of options in the else branch?

[pcorpet]

Can someone help me with the failing test please?

[webpack-bot]

@pcorpet Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@sokra Please review the new changes.

[webpack-bot]

@pcorpet The most important CI builds failed. This way your PR can't be merged.

Please take a look at the CI results from azure (1 errors / 0 warnings) and appveyor (success) and fix these issues.