add defaultTheme prop to specify default theme if no previous value stored

[Yahkob]

I'm not sure if this is too big of a change but would love to have a way to provide an optional prop to show GraphiQL in a specific theme.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 238edcd53cc896e97a25246dbeb77083ed5d37eb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@graphiql/react	Minor
graphiql	Minor
@graphiql/plugin-code-exporter	Major
@graphiql/plugin-explorer	Major

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The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: dimaMachina / name: Dimitri POSTOLOV (de776dacf80d824531c5f2ebb9aca63bc3023c3b, 238edcd53cc896e97a25246dbeb77083ed5d37eb)
• :white_check_mark: login: Yahkob / name: Jake Harclerode (4299163e1d0ac45ae904ec008e2425bc636fd838)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.29%. Comparing base (959ed21) to head (238edcd). Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3619   +/-   ##
=======================================
  Coverage   67.29%   67.29%           
=======================================
  Files         121      121           
  Lines        7017     7017           
  Branches     2268     2244   -24     
=======================================
  Hits         4722     4722           
  Misses       2278     2278           
  Partials       17       17           

Files	Coverage Δ
packages/graphiql-react/src/theme.ts	59.25% <100.00%> (ø)
packages/graphiql/src/components/GraphiQL.tsx	78.41% <100.00%> (ø)