Fix errors when using with Vue 3

[aklinker1]

Summary

This closes #217

• Updated plugin structure to use install method to fix TS error mentioned in the issue
• Added composition helper function, useReCaptcha
• Upgraded to vue@^3.0.0-rc.5

Resources

• https://github.com/vuejs/vuex/blob/4.0/src/store.js
• https://github.com/vuejs/vue-router-next/blob/910634e659be83eabf65b2215e201cdea1c02761/src/router.ts
• https://learnvue.co/2020/03/designing-vue3-plugins-using-provide-and-inject/
• https://vuejs.org/v2/guide/plugins.html

Other Comments

• First time writing a plugin, so I might have over-restructured the main file
• README.md might have some lint errors in new example code, your code style is different than mine but I tried to match it, so let me know if I missed anything
• Couldn't get the demo application working, webpack was complaining about not being able to find the "vue" module, but I did do manual testing on my personal project and both the new useReCaptcha and old usage were working.

[WipeAir]

Hey, sorry for the delay! I haven't had any time yet to look into this topic.
I'll check your PR in the coming days and fix the things you've mentioned.

[aklinker1]

No worries, I understand completely! Most major plugins don't work with vue3 yet either.

[aklinker1]

I'll get the conflicts sorted out sometime soon.

[WipeAir]

Thanks for the PR! Just merged it into the master (and fixed some minor problems).
The demo application is working fine again and the code style matches the original ESLint configuration. :)