Inline constant expressions from mustache tags into template

[benmccann]

Describe the problem

sveltejs/kit#11059 as it stands now would change the output of the @sveltejs/enhanced-img preprocessor to:

<picture><source srcset={"/1 1440w, /2 960w"} type="image/avif" /><source srcset={"/3 1440w, /4 960w"} type="image/webp" /><source srcset={"5 1440w, /6 960w"} type="image/png" /><img src={"/7"} alt="basic test" width=1440 height=1440 /></picture>

This would cause extra output to be inserted into the generated JS:

 	var source = $.child(picture);
	$.attr(source, "srcset", "/1 1440w, /2 960w");
	var source_1 = $.sibling(source);
	$.attr(source_1, "srcset", "/3 1440w, /4 960w");
	var source_2 = $.sibling(source_1);
	$.attr(source_2, "srcset", "5 1440w, /6 960w");

It would be nice not to have these calls and still continue to rely only on the $.template call

Describe the proposed solution

Automatically evaluate and inline any expressions known to have constant values

Alternatives considered

Possibly we can make relative paths not the default in SvelteKit so that we hit this case less often. This optimization may be nice nonetheless not only for the relative paths case, but for other instances where the user may write such code

Importance

nice to have