Renamed view 'Writer' to 'DowncastWriter'

[f1ames]

Suggested merge commit message (convention)

Other: Renamed view Writer to DowncastWriter. Closes ckeditor/ckeditor5#4404.

BREAKING CHANGE: The engine/view/writer module was renamed to engine/view/downcastwriter. See ckeditor/ckeditor5#4404.

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Additional information

Some small adjustments were needed in few other packages, see:

• Adjusted code to 'Writer' to 'DowncastWriter' rename in 'ckeditor5-engine' ckeditor5-clipboard#53
• Adjusted code to 'Writer' to 'DowncastWriter' rename in 'ckeditor5-engine' ckeditor5-image#231
• Adjusted code to 'Writer' to 'DowncastWriter' rename in 'ckeditor5-engine' ckeditor5-link#200
• Adjusted code to 'Writer' to 'DowncastWriter' rename in 'ckeditor5-engine' ckeditor5-list#109
• Adjusted code to 'Writer' to 'DowncastWriter' rename in 'ckeditor5-engine' ckeditor5-table#102
• Adjusted code to 'Writer' to 'DowncastWriter' rename in 'ckeditor5-engine' ckeditor5-widget#51

[f1ames]

I have run unit test for all packages and also checked some manual tests briefly, everything seems to work fine.
For renaming I have used searching for from '@ckeditor/ckeditor5-engine/src/view/writer and view/writer~Writer in all packages plus WebStorm built in refactor/rename mechanism.

[coveralls]

Coverage remained the same at 98.621% when pulling 05d418f on t/1515 into 781d056 on master.