Improve typings, use TypeScript 3.1

[adidahiya]

• Upgrade TypeScript version to 3.1 (this version has been around for quite a while, it's safe to assume your users are on TS 3.1+ at this point, and 3.1 has a lot of bug fixes compared to 3.0)
  • also remove the explicit dependency on typescript, since dtslint pulls it in and you never invoke it directly
• Enable noImplicitAny in tsconfig.json so that you can check you are using types like React.Component correctly
  • install @types/react to get this to work
• Remove redundant re-exports in types/index.d.ts
  • Since DateUtils, LocaleUtils, and ModifiersUtils are all already const values in utils.d.ts which are reexported by export * from './utils';, there is no need to explicitly export them
• Add a README to the types folder to discourage TypeScript users from importing from react-day-picker submodules. This was the reason for the bug in DateTime 3.9.2 datePickerCore.d.ts 'LocaleUtils' type reference error palantir/blueprint#3703: we were importing from react-day-picker/types/utils, which had a breaking change in Fix utils TypeScript types #899. I was able to fix our usage by importing from the module root: [datetime] fix: upgrade react-day-picker, clean up type imports palantir/blueprint#3725

[codecov-io]

Codecov Report

Merging #929 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #929   +/-   ##
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  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines         659    659           
  Branches      146    146           
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  Hits          659    659

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[codecov-io]

Codecov Report

Merging #929 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #929   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines         659    659           
  Branches      146    146           
=====================================
  Hits          659    659

---

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 2a3cef0...8305dee. Read the comment docs.

[gpbl]

Thanks you @adidahiya, sorry for the late merge. I'm going fast forward with version 8 which is interely in typescript. I'd love your feedback: #942