Add typescript support

[voodoods]

Proposed changes

Added support for Typescript as requested in #44

In detail:

• installed necessary Typescript and Babel dependencies
• added tsconfig.json
• updated babel.config.js
• replaced .js files with .ts in ./src and ./test
• updated scripts in package.json and added check-types script
• added custom type declaration file
• updated checklist in README.md

Types of changes

What types of changes does your code introduce to Appium?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov-io]

Codecov Report

Merging #53 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #53   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          51     53    +2     
  Branches        3      3           
=====================================
+ Hits           51     53    +2

Impacted Files	Coverage Δ
src/index.ts	100% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 54a97b9...dd57a4b. Read the comment docs.

[chgasparoto]

Thank you very much for this PR @voodoods
Looks very good to me. I'll take a better look at it later this week.

[chgasparoto]

I don't think we should serve the ts file because a simple nodejs app doesn't support it out of the box. Instead, we should add the types property.

[voodoods]

Thanks for pointing this out! This change was unintended, I reverted this. The build process has of course not been touched.

I've also added a types property pointing to a new custom index.d.ts type declaration file. However I have to point a few things out here:

• this is my first attempt wrting a custom file declaration file for a module; I had to do some research first to get it right - so please feel free to double check
• babel's build process won't do any type checking; I've added a new check-types command to use the Typescript compiler manually without emmiting output
• I suggest to create a new issue to automatically apply type-checking to a commit-hook

Other than that: Sorry for the long delay 🙏

[chgasparoto]

We should use a file generated inside the dist folder, in order to always use the latest version of the types.

[chgasparoto]

We should collect only coverage from the ts files from now on.

[chgasparoto]

The index.ts file doesn't have any type declaration at all, we need to add it to properties and methods. The file index.d.ts should be generated based on the definitions of the main class.