Add doc[bool] conditional no-op toggle for with statements

[Tronic]

The Python with statement cannot be made conditional due to syntax. This PR adds another overload for the [] operator with bool argument to act as such toggle. Using doc[True] does nothing and processing after it continues normally. doc[False] returns a NullBuilder that does absolutely nothing with anything done to it, in effect discarding further operations on the same statement and with statements with it.

with doc[is_enabled].strong:
    doc("always added, sometimes strong")

[Copilot]

Pull request overview

This PR adds a conditional “no-op” toggle for the [] operator so doc[True] behaves normally while doc[False] returns a builder-like object that absorbs chained calls and with blocks, enabling conditional tags without conditional syntax around with.

Changes:

• Added NullBuilder (a no-op context manager/builder) to swallow chained operations when a condition is False.
• Extended Builder.__getitem__ to accept bool and return NullBuilder for False (or self for True).
• Added documentation and tests covering conditional tags, chaining behavior, and NullBuilder behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/test_html5tagger.py	Adds tests for doc[bool] conditional behavior and NullBuilder semantics.
README.md	Documents the new “Conditional elements” feature with an example.
html5tagger/nullbuilder.py	Introduces NullBuilder implementation used for the no-op path.
html5tagger/builder.py	Implements bool handling in __getitem__ and wires in NullBuilder.

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