fix: graphql 15 fixes, feature flags, tests

[acao]

• fix graphql 15 related issues in several packages
• introduce build, test & cypress run against graphql@15
• use version driven logic to make tests pass

I think this will fix a longstanding issue, especially glaring with the graphql@16 upgrade, is ensuring legacy support, and even setting the path for an eventual test matrix of graphql versions if we want

without deploy previews it doesn't feel exhaustive, but this is a good first step

[changeset-bot]

🦋 Changeset detected

Latest commit: 3b6a150

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

This PR includes changesets to release 6 packages

Name	Type
codemirror-graphql	Patch
graphiql	Patch
graphql-language-service-interface	Patch
graphql-language-service-server	Patch
graphql-language-service-utils	Patch
graphql-language-service-types	Patch

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[github-actions]

The latest changes of this PR are available as canary in npm (based on the declared changesets):

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[github-actions]

🚀 Deployed on https://61ae52b623f2cd005a85aed2--graphiql-webpack-example.netlify.app

[github-actions]

🚀 Deployed on https://61ae52b95113e0008f9407cd--graphiql-test.netlify.app

[github-actions]

🚀 Deployed on https://61ae53055113e0008f9407d2--monaco-graphql.netlify.app

[codecov]

Codecov Report

Merging #2091 (e5a0de3) into main (2d91916) will decrease coverage by 2.38%.
The diff coverage is 65.98%.

❗ Current head e5a0de3 differs from pull request most recent head 3b6a150. Consider uploading reports for the commit 3b6a150 to get more accurate results

@@            Coverage Diff             @@
##             main    #2091      +/-   ##
==========================================
- Coverage   65.70%   63.32%   -2.39%     
==========================================
  Files          85       79       -6     
  Lines        5106     5279     +173     
  Branches     1631     1665      +34     
==========================================
- Hits         3355     3343      -12     
- Misses       1747     1932     +185     
  Partials        4        4              

Impacted Files	Coverage Δ
packages/codemirror-graphql/src/hint.ts	94.73% <ø> (ø)
packages/codemirror-graphql/src/lint.ts	100.00% <ø> (ø)
packages/codemirror-graphql/src/results/mode.ts	47.05% <ø> (ø)
...kages/codemirror-graphql/src/utils/forEachState.ts	100.00% <ø> (ø)
...ckages/codemirror-graphql/src/utils/mode-indent.ts	0.00% <0.00%> (ø)
packages/codemirror-graphql/src/variables/hint.ts	89.70% <ø> (ø)
packages/codemirror-graphql/src/variables/mode.ts	79.48% <ø> (ø)
packages/graphiql/src/components/QueryEditor.tsx	63.96% <ø> (ø)
packages/graphiql/src/utility/fillLeafs.ts	5.33% <ø> (ø)
...kages/graphiql/src/utility/introspectionQueries.ts	100.00% <ø> (ø)
... and 59 more

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