String: Add .is-empty and .length properties

[task-jp]

Introduce two new methods for string properties in .slint:

• .is_empty(): Checks if a string is empty.
• .length(): Retrieves the length of the string.

These additions enhance functionality and improve convenience when working with string properties.

[ogoffart]

Thanks a lot for the PR.
Code looks good. One thing missing are the docs.

What is the usecase for this?
I think is-empty make sense (even though it can be done with == "")
But I'm wondering about length, because it is not well defined. Currently it is utf8 bytes. But what can one do with this info? Perhaps one want the number of codepoints? Or does one want to get the size in pixel depending on the font?
Given that, I don't feel so good about adding this function.

(I also wonder if it shouldn't be a property instead of a function, like the one on model/array)

[task-jp]

Thanks for your comment.

Code looks good. One thing missing are the docs.

How can I generate the docs like https://docs.slint.dev/latest/docs/slint/reference/primitive-types/#images ?

I've tried https://github.com/slint-ui/slint/blob/master/docs/development.md#documentation but it seems that only rust documentation is generated.

What is the usecase for this?

For length(), I wanted it to calculate password length in LineEdit.

Perhaps one want the number of codepoints?

Your are right. I will figure out how it would be.

(I also wonder if it shouldn't be a property instead of a function, like the one on model/array)

I thought so, then I found that is-float() was implemented as a function. I will check model/array and try to make them as properties.

[task-jp]

I read https://github.com/slint-ui/slint/blob/master/docs/astro/README.md and was able to build the slintdoc.