Fix ESM-CJS compatibility (#36)

[zetlen]

Resolves #36.

• Updates docx version to 8.1.0, which uses package.json exports correctly and supports direct ESM import
• Updates unist-util-visit to current release
• Uses the modern package.json exports field to conditionally export for module formats
• Removes reliance on unstable import assertions (not strictly necessary to resolve import issues #36 but falls in the category of ESM compatibility)

[inokawa]

Thank you for your PR @zetlen !

For unknown reason, docx 8.1.0 seems to generate no result in src/index.spec.ts and obsolete its snapshots. Could you fix it?

[zetlen]

Added a fix for an unexpected regression.

It looks like version 8.1.0 of docx has a regression where it'll return a Uint8Array instead of a Buffer if it can't tell it's running in Node. The intended behavior is documented here. I'll open the bug in docx when I can. I don't have the time to make a minimal repro for that, unfortunately.

[zetlen]

Whoops, forgot to actually update the snapshots! Stand by

[inokawa]

LGTM
Thanks!