feat(@nestjs/graphql): add conditional exports for browser shim

[yogeshwaran-c]

PR Checklist

• The commit message follows our guidelines
• Tests for the changes have been added (n/a for package.json change; exports verified manually via require.resolve + conditions)
• Docs have been added / updated (n/a for package.json metadata)

Issue

Closes #3468

What is the new behavior?

Adds a proper "exports" map (plus matching top-level "browser" / "react-native" fields for legacy tooling) to @nestjs/graphql's package.json so the browser shim is picked up automatically, without consumers having to configure webpack/Metro/Vite aliases or resolver overrides.

Conditional resolution for the main entry (@nestjs/graphql):

• types -> ./dist/index.d.ts
• browser -> ./dist/extra/graphql-model-shim.js
• react-native -> ./dist/extra/graphql-model-shim.js
• node (nested) -> ./dist/index.js (types/require/import/default)
• require / import -> ./dist/index.js
• default -> ./dist/extra/graphql-model-shim.js (safe fallback for non-Node environments)

Additional subpaths are preserved for back-compat:

• @nestjs/graphql/plugin -> ./plugin.js (with proper types pointing at ./dist/plugin/index.d.ts)
• @nestjs/graphql/package.json -> explicit passthrough
• @nestjs/graphql/* -> wildcard passthrough so existing deep imports such as @nestjs/graphql/dist/extra/graphql-model-shim.js continue to work

The existing main / types fields are kept to preserve compatibility with older tooling that does not understand exports. The shim's documentation comment is also updated to describe the new automatic behavior while keeping the legacy webpack-alias recipe for anyone on older setups.

Resolution verified locally via node --conditions=browser -e "require.resolve('@nestjs/graphql')" (resolves to the shim) and default node -e "require.resolve('@nestjs/graphql')" (resolves to dist/index.js). yarn build passes across all packages (graphql, apollo, mercurius).