Fix source maps generation for the UMD build.

[filipsobol]

Suggested merge commit message (convention)

Fix (build-tools): Fixed source maps generation for the UMD build. Fixes ckeditor/ckeditor5#16984.

Feature (build-tools): Introduced a new loadSourcemaps plugin for loading source maps of external dependencies.

[coveralls]

coverage: 90.797%. remained the same
when pulling 4ad052e on fix-source-maps-in-umd-build
into 6f5a467 on master.

[pomek]

I wouldn't say that UMD CKEditor 5 build works when preparing using changes from this PR:

 at get parent (ckeditor5.js:5:175789)
    at Wh._validate (ckeditor5.js:5:266719)
    at Ju.<anonymous> (ckeditor5.js:5:350416)
    at Ju.fire (ckeditor5.js:5:6572)
    at <computed> [as applyOperation] (ckeditor5.js:5:38783)
    at Pu.insert (ckeditor5.js:5:328486)
    at Object.callback (main.js:51:11)
    at Ju._runPendingChanges (ckeditor5.js:5:355920)
    at Ju.change (ckeditor5.js:5:352975)
    at main.js:48:16

So, it's the same as I reported via a private channel.

[pomek]

PS: We miss an issue that we could use as a reference to explain the problem. The proposed commit message does not explain the reason for the change.

[filipsobol]

I added two UMD builds to trigger the ckeditor-duplicated-modules error.

Before changes:

Uncaught CKEditorError: ckeditor-duplicated-modules
Read more: https://ckeditor.com/docs/ckeditor5/latest/support/error-codes.html#error-ckeditor-duplicated-modules
    at ckeditor5.js:5:5295
    at ckeditor5.umd.js:4:77
    at ckeditor5.umd.js:5:3

After changes:

Uncaught CKEditorError: ckeditor-duplicated-modules
Read more: https://ckeditor.com/docs/ckeditor5/latest/support/error-codes.html#error-ckeditor-duplicated-modules
    at version.ts:162:8        <-----------------------
    at ckeditor5.umd.js:4:77
    at ckeditor5.umd.js:5:3

[pomek]

After reinstalling dependencies it looks good:

position.ts:198 Uncaught CKEditorError: model-position-path-incorrect {"position":{"root":"main","path":[15,3],"stickiness":"toNone"}}
Read more: https://ckeditor.com/docs/ckeditor5/latest/support/error-codes.html#error-model-position-path-incorrect
    at get parent (position.ts:198:11)
    at Wh._validate (insertoperation.ts:113:39)
    at Ju.<anonymous> (model.ts:96:14)
    at Ju.fire (emittermixin.ts:241:31)
    at <computed> [as applyOperation] (observablemixin.ts:281:17)
    at Pu.insert (writer.ts:241:14)
    at Object.callback (main.js:51:11)
    at Ju._runPendingChanges (model.ts:1109:59)
    at Ju.change (model.ts:234:17)
    at main.js:48:16

[pomek]

LGTM.

[pszczesniak]

Tested - works as it should.