chore: alterative functional templating syntax

[paoloricciuti]

Note, this PR is against #15538

You can use this playground link if you want to play around with the output.

It's implementing a version of the comment from @Rich-Harris #15538 (comment) that moves the logic to invoke the function to the runtime instead of build time. Basically instead of actually creating the body of an arrow function this create a structure that template_fn can convert to a series of functional calls. Specifically the structure looks like this

type TemplateStructure = {
    /** the element name */
    e: string, 
    /** if defined the custom element that the element extends */
    is?: string,
    /** all the attributes */
    p: Record<string, string>,
    /** all the children */
    c: Array<TemplateStructure>,
} | null | string | [string];

the function accepts an array of TemplateStructure and follows the following convention:

• If it's undefined or an array it's a comment (if it's an array with a string inside it's the data of <!--data--> otherwise it's an empty comment...this allow us to save bytes because 99% of the times we can actually just send undefined which means an array with an empty spot eg. `[,,"there are two comments before me"];
• if it's a string then it's a text node
• if it's an object we create the actual element, append all the attributes and append all the children

I've opened another PR for a couple of reason:

1. i was not able to find a "shorter" data structure that could represent everything but i'm open to ideas
2. we have the eternal debate between data sent over the wire (in the original pr) vs CPU cycles (in this PR)...we just need to choose what we want.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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• This message body should clearly illustrate what problems it solves.x

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15599

[paoloricciuti]

I'm gonna merge this since we definitely will go either with this or something similar