fix: add missing newline after inline table in serialization

[daniel7an]

Summary

Fixes #440

When an inline table without a trailing newline was followed by a new key-value pair added programmatically, the serialization would concatenate them on the same line:

v = tomlkit.loads('[x]\na.b = {}')
v['x']['c'] = 3
print(tomlkit.dumps(v))
# Before: a.b = {}c = 3
# After:  a.b = {}
#         c = 3

Changes

• tomlkit/container.py: In _render_table, added a newline check before rendering simple items when the preceding content doesn't end with a newline.
• tests/test_toml_document.py: Added regression test covering dotted keys, non-dotted keys, and trailing newline variants.

Testing

All 284 existing tests pass. New regression test added.