feat: improve error message in case of node starting with for

```templ
templ about() {
	<p><b>Free food and drinks</b> for everyone involved.</p>
}
```

Results in the error:

```
parsing error: for: missing opening brace
```

The error message would be better if it gave an explanation of how to resolve.

```
parsing error: for: missing opening brace, if this is not a for loop, write `{ "for" }` to escape the Go keyword
```
