Improve typescript declarations

[romansp]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Improves typings

What is the current behavior? (You can also link to an open issue here)

1. typings for options for install method are inlined, making them very hard to use in app feat: added typings for vue-analytics #258 (comment)
2. library function exports e.g. event, exception, time, etc. are missing, so it's impossible to import them
3. ecommerce types are missing

What is the new behavior (if this is a feature change)?

1. extracted option typings for install method
2. added library functions into exports
3. added typings for ecommerce.

Does this PR introduce a breaking change?
It might break compilation for those who already use typescript together with vue-analytics.

Please check if the PR fulfills these requirements

• The commit message follows semantic-release guidelines
• Fix/Feature: Docs have been added/updated
• Fix/Feature: Tests have been added; existing tests pass

Other information:
Let me know if those typings for ecommerce are too much.

I also added vuex into devDependencies to pull Store<T> type from there and have better declarations for analyticsMiddleware.

[MatteoGabriele]

thanks a lot @romansp 🙏

[MatteoGabriele]

🎉 This PR is included in version 5.20.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀