add locale configuration for docx export

[Ovgodd]

Title
Add configurable locale for DOCX export (default: en-US)

Description
This PR introduces the ability to configure the default language of the generated .docx files via a new optional locale parameter in createDefaultDocumentOptions.

issue 1896

Details

• Added locale?: string parameter to allow consumers of the library to set the document’s default language in the styles.xml metadata (<w:lang w:val="..."/>).
• If no locale is provided, the default is set to en-US.
• This change improves accessibility: screen readers rely on the document’s language metadata to pronounce words correctly.
• Implementation replaces the existing language in styles.xml while preserving other attributes.

Example usage

const exporter = new DOCXExporter(schema, mappings);
await exporter.toBlob(blocks, { locale: "fr-FR" });

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