Expose String.validate_identifier method

[dalexeev]

Expose the validate_identifier method to the public API since validate_node_name is already exposed. validate_identifier can be useful for generating identifiers in EditorPlugins, EditorScripts, and other user code.

Changed behavior of validate_identifier to be consistent with validate_node_name and usability:

1. Invalid characters are removed, not replaced with _.
2. Numeric strings are prefixed with _ rather than replacing the first digit with _.

Changed/added test cases and documentation.

Fixed comment: #67018 (comment).

[akien-mga]

This implementation sounds much slower as you're recreating a string char by char each time, when in the vast majority of cases identifiers are valid and should simply be reused as is.

Could you benchmark it on both valid and invalid identifiers of different lengths?

[dalexeev]

Perhaps it's better to copy valid substrings, like in String::_camelcase_to_underscore (link)?

String new_string;
int start_index = 0;

for (int i = 1; i < this->size(); i++) {
    // ...

    if (/* current char is invalid */) {
        new_string += this->substr(start_index, i - start_index);
        start_index = i;
    }
}

new_string += this->substr(start_index, this->size() - start_index);

If the string is already a valid identifier, then it will be copied entirely 1 time.