refactor(compile-vapor): skip unnecessary closing tags in templates

[cyyynthia]

This PR improves the compiled code for Vapor components, by omitting unnecessary HTML close tags resulting in a smaller output. Resolves a TODO specified in code comments.

Closing tags are skipped when the element is the last child of its parent. Even when the parent isn't the last element of the grandparent except for formatting elements (as defined by the HTML spec) since they cause issues with reconstruction, and when the parent has the same tag (since the parent's closing tag would actually close the child).

If the above description isn't clear enough, perhaps this table (+ the tests) will shed some light on the compiler's behavior.

Vue template	Generated template
<div><span/></div>	<div><span>
<div><span/><span/></div>	<div><span></span><span>
<div><p><span/></p><span/></div>	<div><p><span></p><span>
<div><p><b/></p><span/></div>	<div><p><b></b></p><span>
<div><div><span/></div><div/></div>	<div><div><span></div><div>
<div><div><div/></div><div/></div>	<div><div><div></div></div><div>

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Size Report

Bundles

File	Size	Gzip	Brotli
compiler-dom.global.prod.js	85.2 kB	29.9 kB	26.4 kB
runtime-dom.global.prod.js	108 kB	40.5 kB	36.4 kB
vue.global.prod.js	166 kB	60.5 kB	53.8 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	48 kB	18.8 kB	17.1 kB
createApp	57 kB	21.9 kB	20 kB
createApp + vaporInteropPlugin	88.5 kB (-6.92 kB)	33.2 kB (-1.94 kB)	30 kB (-1.75 kB)
createVaporApp	31.7 kB (-8.88 kB)	12.4 kB (-2.65 kB)	11.3 kB (-2.38 kB)
createSSRApp	61.4 kB	23.7 kB	21.6 kB
defineCustomElement	63.1 kB	23.8 kB	21.7 kB
overall	72.1 kB	27.3 kB	24.9 kB

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commit: 369b70b

[edison1105]

Thanks for your PR. Could you please add some tests in dom/template.spec.ts?

[cyyynthia]

I've added tests in fb678dd. 👍🏻

[cyyynthia]

Rebased the changes to solve conflicts 😄