Add dtslint setup to validate type definitions

[kohlmannj]

connects #96, #97, #99, #100, #101, #102

Description

I'm really excited to start using Unistore within a TypeScript project! However, when I installed Unistore and started using its type definitions, I noticed two errors when reviewing preact.d.ts and react.d.ts in Visual Studio Code (using TypeScript v2.9.2): #96, #97

To avoid issues with Unistore's type definitions in the future, I've added an additional test command to validate the type definitions using dtslint.

Q&A

Why "TypeScript Version: 2.2"? Evidently these type definitions as written rely upon some feature introduced in TS 2.2. dtslint recommended this addition when I tried running it without the // TypeScript Version: 2.2 comments:

dtslint Error without TypeScript Version Comments

Test with 2.1
Error: /Users/206837/Sites/kohlmannj/unistore/index.d.ts:13:19
ERROR: 13:19  expect  Compile error in typescript@2.1 but not in typescript@2.2.
Fix with a comment '// TypeScript Version: 2.2' just under the header.
Cannot find name 'object'.
ERROR: 21:41  expect  Compile error in typescript@2.1 but not in typescript@2.2.
Fix with a comment '// TypeScript Version: 2.2' just under the header.
Cannot find name 'object'.

Why are some of dtslint's rules disabled in tslint.json? My goal in this PR is to have dtslint's error output contain exactly these two errors. Future PRs might improve the type definitions by re-enabling these disabled rules.

Why are preact and react in tsconfig.json’s paths config? First, I set the path to unistore so that dtslint resolves index.d.ts for the module name ”unistore”. (dtslint recommends doing this in their docs.) That caused ”preact” and ”react” to incorrectly resolve to the two local preact.d.ts and react.d.ts files, however. Hence, I explicitly set paths to them (though I wonder if the ”react” path should be node_modules/react instead?).

Why is CI failing for this PR? CI fails right now because of the added dtslint invocation in the test script. As described above, this is symptomatic of now testing something previously untested. We can resolve this by merging fixes for both #96 and #97.

Notes

I'm not sure what's wrong with the Preact definitions in #96, so I cannot help there.

However, #97 was super easy, so I've opened #99 as well.