Add pair selecting support for cond forms

[jramosg]

What has changed?

This PR adds pair selecting behavior for cond forms

Implementation

• Added conditionalForms constant to identify forms that have test/expr pairs
• Added getConditionalFormPairOffset() function to handle offset requirements
• Modified isInPairsList() to detect cond forms in lists
• Updated currentSexpsRange() to use pairOffset when calculating pair indices
• Added comprehensive tests for cond form pair selection

Status

Currently implemented:

• cond - fully working with tests
• case and condp should be included with other issue/PR because they can be different (defaults, result-expr or result-fn, etc.)

Why This Approach

Rather than hardcoding offset values directly in the pairing logic, we:

1. Created a separate getConditionalFormPairOffset() function that determines the offset dynamically based on the form type. This makes easier to add more conditional forms in the future
2. This keeps the main currentSexpsRange() logic clean and maintainable

Alternative Considered

Could have created separate functions for each form type, but this would duplicate logic. The offset approach is more maintainable.

Fixes #2991

My Calva PR Checklist

I have:

• Read How to Contribute.
• Directed this pull request at the dev branch. (Or have specific reasons to target some other branch.)
• Made sure I have changed the PR base branch, so that it is not published. (Sorry for the nagging.)
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  • Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.
• Figured if anything about the fix warrants tests on Mac/Linux/Windows/Remote/Whatever, and either tested it there if so, or mentioned it in the PR.
• Added to or updated docs in this branch, if appropriate
• Tests
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
• Formatted all JavaScript and TypeScript code that was changed. (use the prettier extension or run npm run prettier-format)
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