feat: Upgrade to TypeScript 4 and remove deprecated API

[diegohaz]

Closes #109
Closes #117

BREAKING CHANGE: Types now depend on TypeScript v4.0.

BREAKING CHANGE: The deprecated function/object API has been removed.

Before:

import createUseContext from "constate";
const useCounterContext = createUseContext(useCounter);
<useCounterContext.Provider>
  ...
</useCounterContext.Provider>

After:

import constate from "constate";
const [CounterProvider, useCounterContext] = constate(useCounter);
<CounterProvider>
  ...
</CounterProvider>

[codecov-commenter]

Codecov Report

Merging #118 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #118   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           54        29   -25     
  Branches         7         4    -3     
=========================================
- Hits            54        29   -25     

Impacted Files	Coverage Δ
src/index.tsx	100.00% <100.00%> (ø)

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