avm2: Create a FormatSpans for the text block's content element in createTextLine

[Lord-McSweeney]

Description

Instead of accessing the text of the content element using a getter, create a full FormatSpans for the content element. Currently we discard most of the useful information from it, so this PR shouldn't significantly affect real-world content much. However, this PR does also fix code that uses GroupElement to create lines, so it makes some content (e.g. w2w) render correctly now.

Testing

I'm not adding tests in this PR as it's just an intermediate step to better FTE support. In my next 1-2 PRs, I'll add comprehensive tests for FTE and TLF text rendering.

Checklist

• I, a human, have self-reviewed this PR and fully understand the changes within.
• I have made or updated tests where possible.
• All of my commits are properly scoped, compile successfully, and pass all tests.
• This PR does not make sense to split up into smaller PRs.
• An LLM was involved in the authoring of this code.