…d extended diagnostic

Replaces the `node.keySpan.toString().startsWith('attr.')` string allocation in the `suffixNotSupported` extended template check with an O(1) `node.type === BindingType.Attribute` enum comparison.

The diagnostic message string is also extracted to a module-level constant so it is created once at module load time instead of on every diagnostic emit.

Additionally, this change adds missing test coverage for the `.%` and `.em` suffixes, as well as for a plain `attr.` binding without a style suffix.

Measured with a 100-iteration microbenchmark before and after the change (MacBook Pro 2018, Intel CPU):

```ts
const start = performance.now();
for (let i = 0; i < 100; i++) {
  new ExtendedTemplateCheckerImpl(templateTypeChecker, program.getTypeChecker(),
    [suffixNotSupportedFactory], {}).getDiagnosticsForComponent(component);
}
console.log((performance.now() - start) / 100, 'ms/iter');
```

Before: `~0.24 ms/iter`
After: `~0.14 ms/iter` (~40% faster)