Subpath exports so transports are opt-in

[llbbl]

Deferred from #5 (see PR #12).

package.json exposes a single "." export, so every consumer loads the full barrel — including the transports and their SDKs — even for a pure Markdown→DOCX conversion.

GoogleDriveTransport mitigated its share by loading @googleapis/drive lazily inside upload(), but that is a workaround for the packaging shape rather than a fix.

Tasks

• Add subpath exports: ., ./google, ./sharepoint (types + import conditions for each).
• Decide whether the barrel keeps re-exporting the transports, or whether they become subpath-only (breaking; would need a major/minor call).
• Re-check the google-auth-library pin. It is a devDependency pinned to exactly 10.5.0 because googleapis-common pins it exactly; a @googleapis/drive bump can reintroduce two copies and a Types have separate declarations of a private property 'redirectUri' failure on OAuth2Client.

Relates to #7.