Expose Packr flags + TypeScript fixes

[endel]

Hi there,

This PR exposes the following flags:

• REUSE_BUFFER_MODE
• RESET_BUFFER_MODE
• RESERVE_START_SPACE

It also exposes these to TypeScript:

export class Packr extends Unpackr {
	// ...
	offset: number;
	useBuffer(buffer: Buffer): void;
	clearSharedData(): void;
}

Let me know if something's missing so I can improve this PR.

Thank you for the great work on this lib @kriszyp, cheers! 💙

[endel]

Update: added the missing TypeScript types for the options argument of both .pack() and .unpack() methods: endel@7f77495

[endel]

Update: exposed Packr.offset to TypeScript 🙌

[kriszyp]

Thank you for the PR!