Cache fflate dynamic import to avoid redundant imports on subsequent exports

[Copilot]

Addresses feedback from #16561 to cache the dynamically imported fflate module at the class level.

Changes:

• Add static fflateModulePromise property to cache the import promise
• Add getFflateModule() method that returns cached promise or creates new import
• Promise caching pattern ensures thread-safety for concurrent export operations

private static fflateModulePromise: Promise<typeof import('fflate')> | null = null;

private static getFflateModule(): Promise<typeof import('fflate')> {
    if (!IgxExcelExporterService.fflateModulePromise) {
        IgxExcelExporterService.fflateModulePromise = import('fflate');
    }
    return IgxExcelExporterService.fflateModulePromise;
}

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

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