fix(graphql-language-service-server): allow getDefinition to work for unions

[AaronMoat]

Fixes: #2575

Fixes the issue where a schema like

union X = A | B
type A { x: String }
type B { x: String }
type Query { a: X }

won't allow you to click through to X.

Also fixes the case where this is split across two files (e.g. type Query { a: X } is in one file and the rest are in another)

[changeset-bot]

🦋 Changeset detected

Latest commit: ded2d72

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

This PR includes changesets to release 10 packages

Name	Type
graphql-language-service-server	Patch
graphql-language-service	Patch
graphql-language-service-cli	Patch
vscode-graphql	Patch
cm6-graphql	Patch
codemirror-graphql	Patch
@graphiql/react	Patch
graphiql	Patch
monaco-graphql	Patch
@graphiql/plugin-explorer	Patch

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• ✅ login: AaronMoat / name: Aaron Moat (c8f0c65, c81521c)

[AaronMoat]

@acao this might be one in your area of interest? :)

[acao]

thank you @AaronMoat ! this is a very helpful improvement

[acao]

@AaronMoat this improvement will be reflected in monaco-graphql and graphiql@3 when we introduce definitions as well!