Remove abstract modifier from CommandLineParser and eliminate redundant _actions field

[Copilot]

CommandLineParser can now be instantiated directly without subclassing, useful for testing and simple CLI scenarios. The redundant _actions array field has been removed in favor of deriving the list from _actionsByName.values().

Changes

• Removed abstract modifier from CommandLineParser class - no abstract methods exist, direct instantiation is safe
• Eliminated _actions array field - redundant storage for action list
  • actions getter now returns Array.from(this._actionsByName.values())
  • Iterations use _actionsByName.values() directly
• Updated JSDoc - changed "abstract base classes" to "base classes"
• Added test verifying direct instantiation

Example

// Before: Required subclassing
class MyParser extends CommandLineParser {
  constructor() {
    super({ toolFilename: 'mytool', toolDescription: 'My tool' });
  }
}

// After: Direct instantiation now works
const parser = new CommandLineParser({
  toolFilename: 'mytool',
  toolDescription: 'My tool'
});
parser.addAction(myAction);

Backward compatible - existing subclasses continue to work unchanged.

Original prompt

On CommandLineParser.ts:57-57, Remove the abstract modifier from CommandLineParser. It is useful to be able to instantiate the class directly, e.g. in test classes, and no critical functionality (notably addAction) is restricted to subclasses.
Once it has been removed, generate a rush change file for ts-command-line as a patch.

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