feat(build): Add exports to make package usable as a library

[AntonioVentilii]

Motivation

As per @marceljuenemann's PR #194:

Currently there are no .d.ts files in the published package. This means you can't use this package as a library.

Thank you for that!!!

[Copilot]

Pull request overview

This PR aims to make @dfinity/ic-pub-key usable as a TypeScript/JavaScript library by publishing TypeScript declaration files and defining explicit package entrypoints via exports.

Changes:

• Enable TypeScript declaration (.d.ts) and declaration map generation in tsconfig.json.
• Update package.json to publish dist/, point main/types at dist/, and add exports for root and selected subpaths (./ecdsa, ./schnorr, ./signer).

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tsconfig.json	Turns on declaration + declaration maps so the build emits typings into dist/.
package.json	Publishes dist/ as the package output and adds conditional exports for library consumption.

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