feat: Deprecate old function/object API

[diegohaz]

This PR deprecates old APIs. There's no breaking changes, only deprecation warnigns.

Migration instructions

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-io]

Codecov Report

Merging #101 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #101   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          43     54   +11     
  Branches        5      7    +2     
=====================================
+ Hits           43     54   +11

Impacted Files	Coverage Δ
src/index.tsx	100% <100%> (ø)	⬆️

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