Support <!-- no-format --> HTML comment syntax for markdown code blocks

[Copilot]

Not all markdown renderers support the existing inline no-format syntax (```swift no-format). This adds support for an HTML comment on the line immediately preceding the code block as an alternative:

<!-- no-format -->
```swift
func example()
{
    doSomething()
}
```

Changes

• Sources/CommandLine.swift — parseCodeBlocks now checks the line before an opening fence for <!-- no-format --> and merges it into the block's options. Combines with any inline options if both are present.
• README.md — Documents both syntaxes in the "Code block options" section.
• Tests/ParsingHelpersTests.swift — Three test cases: basic HTML comment syntax, combined with inline options, and a blank line between comment and fence (should not apply).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.45%. Comparing base (dac0c06) to head (1e8a047).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2642   +/-   ##
========================================
  Coverage    95.44%   95.45%           
========================================
  Files          177      177           
  Lines        27073    27084   +11     
========================================
+ Hits         25841    25853   +12     
+ Misses        1232     1231    -1     

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