feat: export mergeThemes and add examples

[levithomason]

This PR exports mergeThemes which allows consumers to merge themes at the top of their application.

import { mergeThemes } from '@stardust-ui/react'

<Provider theme={mergeThemes(themes.teams, { /* my overrides */})}>
  <MyApp />
</Provider>

[codecov]

Codecov Report

Merging #285 into master will increase coverage by 0.98%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   90.77%   91.76%   +0.98%     
==========================================
  Files          41       41              
  Lines        1334     1335       +1     
  Branches      194      194              
==========================================
+ Hits         1211     1225      +14     
+ Misses        120      107      -13     
  Partials        3        3

Impacted Files	Coverage Δ
src/components/Provider/Provider.tsx	88.88% <ø> (+28.88%)	⬆️
src/index.ts	100% <100%> (ø)	⬆️

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