fix(elements): add export for DefaultMergeStrategy - 20.1.x

[IMinchev64]

Related to #16024

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

[Copilot]

Pull Request Overview

This PR adds the missing export for DefaultMergeStrategy in the public API of the igniteui-angular-elements package, addressing issue #16024.

• Added import of DefaultMergeStrategy from the merge-strategy module
• Exported DefaultMergeStrategy with the IgcDefaultMergeStrategy alias

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[MayaKirova]

@IMinchev64 @dkamburov @skrustev Extending the default merge strategy in react works fine now, however when building from this branch and bundling the packages for react manually it seems that merging breaks:

Not sure if it's an issue with missing styles or maybe something else that caused issue with the size of the merged cells. Either way it needs more investigation.