WIP: Add unit test for template literal concatenation (Issue #4148)

[rahulmansharamani14]

This draft PR starts addressing Issue #4148 (Concatenated template literals are not merged in ADVANCED mode).

What’s in this draft:

• Adds a minimal failing unit test in PeepholeFoldConstantsTest that asserts two untagged template literals joined by + should be merged into a single template literal.

Next steps (planned):

• Update PeepholeFoldConstants to fix existing failing tests
• Add more tests to cover more scenarios, including:
  - string + template literal (with any order)
  - string + string (already working)
  - template literal + template literal (template literal can include referenced variable or valid HTML)

Opening early for review to validate direction before proceeding.

[rahul-kamat]

Hey Rahul! I think your approach sounds great, thanks for finding this issue, looking forward to seeing your planned out next steps!