Addresses #106

Changes made

Used ts-migrate to perform the migration

This is a first pass migration where all the files were changed from using a .js extensionto using a .ts extension with the exception of index.js (which is a noop entry file).

I opted to use the reignore option in ts-migrate in order to have the project be in a compilable state so that we can add types in over time rather than needing to introduce them all within this pull request.

Introduced a tsconfig.json file in the root directory and in the __tests__ directory.

The one in the __tests__ directory extends from the one in the root directory, but is set to not emit JS files.

The configuration of the root directory tsconfig.json is the same one as that found in netlify-plugin-gatsby, but jest was added to the types property in order to ensure that the types associated with Jest are correctly picked up by the Typescript compiler.

Misc. changes

• Small code tweaks in order to remove ts-expect-error statements that were added by ts-migrate
• Updated commands to use the Typescript compiler in package.json

Testing plan

• Existing tests pass
• Linking these changes to netlify-plugin-gatsby and able to run demo Gatsby project in that project successfully