Qute: demand valid "property" identifiers

[mkouba]

Currently, we don't specify any requirements except for the starting char of a tag: https://quarkus.io/guides/qute-reference#identifiers.

Our starting definition could be: a valid identifier is a sequence of characters that contains digits, alphabet characters and underscore.

We should also validate all expressions.

[mkouba]

Hm, we'll need to be more lenient because we already use non-standard characters in built-in helpers, e.g. elvis operator: {name ?: 'John'} is translated to name.?:('John'). We could use digits, letters, _ and $ to cover all Java identifiers and add ? and : for now and see how it goes. Alternatively, we could permit any non-whitespace chars.

[mkouba]

I think we should permit any non-whitespace chars but encourage users to only use valid Java identifiers.