Provide support for plural forms i18n, make more generic tools

[ma2ciek]

Suggested merge commit message (convention)

Feature: Provided support for plural forms i18n. Closes ckeditor/ckeditor5#6526. Closes ckeditor/ckeditor5#988.

MINOR BREAKING CHANGE: Omitting the language property in the CKEditorWebpackPlugin will not have any effect from now. This means that in both cases only the main language (language) will be added to the main bundle and translations for other languages will be saved in separate files.
MAJOR BREAKING CHANGE: The translation process no longer creates short ids for message strings. From now, the source code will not be changed by the translation process, translations for the main language will be added to the bundle(s) and translations for other languages will be outputted to separate executable JS files.

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Additional information

The t() function accepts now a string or an object with the following properties:

• string (required)
• id - used to identify ambiguous translations
• plural - used when messages should support plural forms

This PR touches (but not resolves directly)

• Translations in third party plugins ckeditor5#1471
• Allow passing your own translations when building CKEditor 5 ckeditor5#763

For example – encountered issues, assumptions you had to make, other affected tickets, etc.

Checklist

• CKEditor 5 manual tests work with new CKEditorWebpackPlugin
• CKEditor 5 builds work with various options of new CKEditorWebpackPlugin
• Integrations work with new CKEditorWebpackPlugin
• Docs update for the whole translation process
• Docs for the end-user
• Generating CKE5 docs

[coveralls]

Coverage decreased (-0.6%) to 91.052% when pulling 24a4209 on i/6526 into 1841693 on master.

[ma2ciek]

I've found an issue with generating the ~getPluralForm~ function from the Tx rules for some languages. Debugging... False alarm.

[mlewand]

Merged the master branch.

[mlewand]

TC:

1. cd ckeditor5 # go to ckeditor 5 project
2. yarn link @ckeditor/ckeditor5-dev-webpack-plugin # assuming that you already executed `yarn link` in ckeditor5-dev/packages/ckeditor5-dev-webpack-plugin
3. yarn run manual -f table

I get following error:

[CKEditorWebpackPlugin] Error: Too many JS assets has been found during the compilation. You should add translation assets directly to the application from the `lang` directory or use the `allowMultipleJSOutputs` option to add translatiolanguage to all assets.
ns for the main language to all assets.

[mlewand]

Building the docs also throws an error:

Building 127 snippets...
Building snippets for language "en": 40% (building)[CKEditorWebpackPlugin] Error: No JS asset has been found during the compilation. You should add translation assets directly to the application from the 'lang' directory.
Building snippets for language "en": 89% (module assets processing)[CKEditorWebpackPlugin] Error: Too many JS assets has been found during the compilation. You should add translation assets directly to the application from the `lang` directory or use the `allowMultipleJSOutputs` option to add translations for the main language to all assets.
Build completed in 60.019s

Building snippets for language "multi-language": 20% (building)[CKEditorWebpackPlugin] Error: No JS asset has been found during the compilation. You should add translation assets directly to the application from the 'lang' directory.
Build co

[ma2ciek]

These warnings should be fixed now after merging ckeditor/ckeditor5#6654.

[mlewand]

All right, works well now.