feat(hx-live): support boolean attribute binding with :?attr (#4019)

[AndrewGit25]

Description

Adds support for an explicit boolean attribute binding modifier—:?<attr> and its canonical form hx-live:?<attr>—to hx-live.
While hx-live toggles native boolean attributes (disabled, required, etc.) based on truthiness, generic attributes stringify booleans (e.g. :multiple="false" writes multiple="false"). For Web Components (like Lit's @property({ type: Boolean })), Custom Elements, and custom boolean attributes where attribute presence dictates state rather than string value, this caused attributes to remain present when falsy.
Borrowing the ? prefix convention from Lit and the Web Components ecosystem:

• Truthy values (true, non-empty strings, positive numbers) add/retain the attribute via native toggleAttribute().
• Falsy values (false, null, undefined, 0, "") remove the attribute entirely via native toggleAttribute().
  Example:

<!-- Truthy adds multiple="", falsy removes multiple entirely -->
<custom-select :?multiple="q('#count').valueAsNumber > 1"></custom-select>
<!-- Canonical form for build pipelines stripping ':' -->
<div hx-live:?custom-flag="q('#checkbox').checked"></div>

Corresponding issue: Closes #4019

Testing

1. Automated Unit Tests:
   • Added unit tests in test/tests/ext/hx-live.js covering :?attr, hx-live:?attr, and truthy/falsy evaluation rules.
   • Ran npm run test (all 1,739 tests passed across 87 test files with 100% code coverage).
   • Ran npm run check:content (all doc and API checks passed).
2. Manual Testing:
   • Tested interactively via test/scratch/scratch.html with real-time DOM mutation inspection on custom web components and standard elements.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is a new feature discussed with and guided by @MichaelWest22 in Feature Request: hx-live: Support Boolean Attribute Binding (:?attr / hx-live:?attr) for Custom Attributes & Web Components #4019
• I ran the test suite locally (npm run test) and verified that it succeeded