Fix types root in package.json

[cmoesel]

Description: This sets the types root to dist/index.d.ts. Previously, the types root pointed to dist/app.d.ts, which prevented GoFSH from being imported property in other applications.

Testing Instructions:
Repro the problem:

1. Download the attached simple TS project that imports GoFSH: ts-project.zip
2. Unzip it and run npm install
3. Run npm run build and observe compile errors

Test the solution:

1. Check out the branch from this PR
2. Run npm pack to create the module file gofsh-2.3.1.tgz
3. Copy gofsh-2.3.1.tgz to the simple TS project you downloaded
4. In that simple TS project, run npm install --save file:gofsh-2.3.1.tgz
5. Run npm run build and observe that it builds cleanly

Related Issue: Discovered and discussed in FSH VS Code PR #97

[mint-thompson]

Worked perfectly for me: compile errors without the change, no compile errors with the change. Thank you!