Fix inline table separator after append

[cyliu0]

Summary

Fix inline table serialization after appending a new key to a parsed inline table that already preserved explicit comma tokens.

The serializer now tracks whether an emitted key still needs a separator. When a later mutation adds a key without an explicit comma token, as_string() inserts the missing comma before rendering the next key.

Fixes #476.

Tests

• python3 -m pytest tests/test_items.py::test_appending_to_parsed_inline_table_preserves_separator tests/test_items.py::test_deleting_inline_table_element_does_not_leave_trailing_separator tests/test_items.py::test_deleting_inline_table_element_does_not_leave_trailing_separator2 tests/test_items.py::test_items_can_be_appended_to_and_removed_from_an_inline_table -q
• python3 -m pytest tests -q
• git diff --check

[cyliu0]

Hi @frostming, could you please take a look at this PR when you have a chance? It fixes a 0.15.0 regression where appending a key to a parsed inline table can serialize invalid TOML. The PR includes a regression test, and the full test suite passes locally (968 passed).