Formspec: Add alignment styles to label[]

[CrazyladMT]

Fixes #7613

Note

I used GPT-5 to partially assist with these changes.

Falls under section 2.3 in the roadmap

Adds support for aligning text in a label[] element by adding two new styles: halign (horizontal alignment) and valign (vertical alignment)

halign can be either one of the following:

• left
• center
• right

valign can be either one of the following:

• top
• center
• bottom

To do

This PR is a Work in Progress.

• add documentation
• add examples to test_formspec in devtest?

How to test

1. Run the test code below

Test code

core.register_chatcommand("test_labels", {
    func = function(name)
        local fs = [[
            formspec_version[6]
            size[11,9]

            box[0.5,2.8;3,1;#222]
            label[0.5,2.8;3,1;Default]

            box[4,2.8;3,1;#222]
            style_type[label;halign=center;valign=center]
            label[4,2.8;3,1;Center/Center]

            box[7.5,2.8;3,1;#222]
            style_type[label;halign=right;valign=bottom]
            label[7.5,2.8;3,1;Right/Bottom]

            # Reset style
            style_type[label;halign=left;valign=top]

            box[0.5,4.2;3,1;#222]
            style_type[label;halign=left;valign=bottom]
            label[0.5,4.2;3,1;Left/Bottom]

            box[4,4.2;3,1;#222]
            style_type[label;halign=center;valign=top]
            label[4,4.2;3,1;Center/Top]

            box[7.5,4.2;3,1;#222]
            style_type[label;halign=right;valign=center]
            label[7.5,4.2;3,1;Right/Center]

            # Reset again
            style_type[label;halign=left;valign=top]

            label[0.5,5.7;Font size tests:]

            box[0.5,6.2;3,1.2;#222]
            style_type[label;font_size=16;halign=center;valign=center]
            label[0.5,6.2;3,1.2;Size 16]

            box[4,6.2;3,1.2;#222]
            style_type[label;font_size=26;halign=center;valign=center]
            label[4,6.2;3,1.2;Size 26]

            box[7.5,6.2;3,1.2;#222]
            style_type[label;font_size=36;halign=center;valign=center]
            label[7.5,6.2;3,1.2;Size 36]

            # Reset styles at end
            style_type[label;halign=left;valign=top;font_size=16]
        ]]

        core.show_formspec(name, "test:labels", fs)
    end
})

[siliconsniffer]

Could you also support styling of textarea[] elements? This would help with #16953.

[CrazyladMT]

Could you also support styling of textarea[] elements? This would help with #16953.

I totally overlooked that. Will do :)

[CrazyladMT]

Regarding textarea[] styling, should it only be able to be aligned when it is in read-only mode, or edit mode too? I find the latter strange.

[siliconsniffer]

One thing to keep in mind: styling a read-only textarea[] element would only be possible through style-type[] since style[] uses the name for selection.
That being said, I don't think it matters much although I could see centered text in edit mode being used.

[CrazyladMT]

I will leave textarea[] styling for another PR since this PR is about adding styling to labels