Extract rule: template-no-down-event-binding

[NullVoxPopuli]

Split from #2371.

[NullVoxPopuli]

TODO: this rule isn't correct

[johanrd]

Claude Review: comparing against ember-template-lint no-pointer-down-event-binding

Wrong event set. The rule targets mousedown + touchstart, but the original targets mousedown, onmousedown, pointerdown, onpointerdown. touchstart is not part of the original rule — remove it.
Add pointerdown and onpointerdown (they're the reason the original is called no-pointer-down-event-binding).

// current                                                                                                                                                                                                         
const DOWN_EVENTS = new Set(['mousedown', 'touchstart']);                                                                                                                                                          
                                                                                                                                                                                                                   
// should be                                                                                                                                                                                                       
const DOWN_EVENTS = new Set(['mousedown', 'onmousedown', 'pointerdown', 'onpointerdown']);                                                                                                                         
                                                                                                                                                                                                                   
Missing {{action}} modifier handling. The original detects <div {{action this.handler on="mousedown"}}> via the on hash pair. This PR only handles {{on}} modifiers and HTML attributes. Add an {{action}} check:  
                                                                                                                                                                                                                   
// inside the modifier loop:                                                                                                                                                                                       
if (modifier.path.original === 'action') {
  const onPair = modifier.hash?.pairs?.find(p => p.key === 'on');
  if (onPair && DOWN_EVENTS.has(onPair.value.value?.toLowerCase())) {                                                                                                                                              
    context.report({ node: modifier, messageId: 'unexpected' });                                                                                                                                                   
  }                                                                                                                                                                                                                
}                                                                                                                                                                                                                  

Missing case-insensitive matching. The original calls .toLowerCase() before checking the set. Add .toLowerCase() to both the {{on}} event param check and the attribute name check.

Attribute check is fragile. attr.name.replace('on', '') replaces the first occurrence of "on" anywhere in the string, not just the prefix. Use instead:

if (attr.name.startsWith('on') && DOWN_EVENTS.has(attr.name.slice(2).toLowerCase())) {

Error message diverges. The original says "Avoid binding to a pointer down event; bind to a pointer up event instead". The PR recommends click/keydown which is different guidance. Suggest matching the original
message.

Docs should also be updated to match — remove touchstart references, add pointerdown examples, fix the recommended alternatives from click/keydown to mouseup/pointerup.