Wrap up low level placeholder handling into PlaceholderConfig helper

[ianjosephwilson]

• Move low level placeholder operations and constants into placeholder config helper class.
• This is to avoid potential free-threading issues but overall we probably shouldn't be using dynamic module level variables anyways.
• Long term we probably want to share the placeholder configuration across a "group" of parsers but I think this is ok for now.

[pauleveritt]

Thanks for doing this. Once this is merged, I'll switch my aria-testing package (which I use heavily in other packages) to free threading and see if I spot any problems. I'll also put the pytest-freethreaded stuff in that package and keep trying to trigger.

As a note, the only potential (but unlikely) other issue CC found:

🟡 POTENTIAL ISSUE: Mutable Dataclass Children (nodes.py, parser.py)

@dataclass
class OpenTElement:
    tag: str
    attrs: tuple[TAttribute, ...]
    children: list[TNode] = field(default_factory=list)  # Mutable!

Problem: list is mutable. If multiple threads access/modify the same OpenTElement's children, race conditions occur.

Current Safety: These are created per-parse operation and not shared between threads, so likely safe in practice.

Impact: LOW - Only matters if users share parser objects across threads (discouraged pattern)