feat: add ability to preserve ts file extension

[nickmessing]

Description

Add preserveTSExtension boolean configuration that allows .ts file extensions (for native import without transpilation tools)

Related #10490
Prerequisite for eddeee888/graphql-code-generator-plugins#364

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit tests included
• Tested on a production project to not break anything with old config
• Tested on a production project to add .ts extension when flag is present

Test Environment:

• OS: macOS 26.0.1
• @graphql-codegen/cli: latest
• NodeJS: v25.1.0

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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Latest commit: 59a10a9

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[nickmessing]

Should I add the changeset too?