v2: publish typescript declarations

[haltcase]

v2 does not include TypeScript declarations because they aren't being created. This PR enables compilerOptions.declaration to fix that. *.d.ts files are now created in dist and therefore published properly to npm.

Tested with yarn pack, published content is now:

dist/
├── context.d.ts
├── context.js
├── index.d.ts
├── index.js
├── partials.d.ts
└── partials.js
CHANGELOG.md
README.md
package.json

[haltcase]

Hey @alexlafroscia, friendly ping about this PR. I'm not blocked since I vendored the types from the repo into my own, but it would be nice to remove that.

Hope all's well 👋