Update webpack merge

[just-jeb]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

• Custom webpack builder uses webpack-merge@4, with smart strategy as a default.
• The configuration field is called mergeStrategies

Issue Number: N/A

What is the new behavior?

• Custom webpack builder uses webpack-merge@5, mergeWithRules method, with some predefined configuration as a default. Tried to keep it as close as possible to the smart strategy.
• The configuration field is called mergeRules

Does this PR introduce a breaking change?

[x] Yes
[ ] No

The migration path is added to MIGRATION.md

[autoapproval]

Approved 👍