perf - do not use class for Codicon

[bpasero]

This speeds up codicons.js module load time 100% for me:

Before

After

[bpasero]

@aeschli fyi I decided here to name fontCharacter actually codiconFontCharacter to be able to implement a good isCodicon method that can check on that. Previously we were able to do instanceof Codicon, but with this change we no longer can. If we kept fontCharacter, there is a chance we would wrongly match on an object that has both id and fontCharacter. Since I found a few more types with fontCharacter I decided to go this path, but please verify that is OK.

[bpasero]

Hm, I wonder if we can even get rid of namespace and use something along the lines of:

export const Codicon = {

	// built-in icons, with image name
	'add': makeCodicon('add', '\\ea60'),

Because looking at microsoft/TypeScript#51387, there seems to be additional lookup cost when using namespace. The only then to keep in mind is that you cannot reference another Codicon from within that object literal as we do for some icons ('treeItemExpanded': makeCodicon('tree-item-expanded', chevronDown.codiconFontCharacter))

[bpasero]

Better done in #171254