Add support for converting existing collection-initializers over to collection expressions.

[CyrusNajmabadi]

Part of #69132

Draft until #69280 goes in.

This means code like new List<int> { 1, 2, 3 } will get converted to [1, 2, 3]. We also detect this in the long-statement form like:

List<int> result = new List<int>();
result.Add(1);
result.Add(2);
result.Add(3);

And offer a similar conversion. Additionally we support converting things like:

List<int> result = new List<int>();
result.Add(1);
result.Add(2);
foreach (var v in x)
    result.Add(v);

to: [1, 2, .. v], and so on.

Note: this also works when the initializer already exists. For example:

List<int> result = new List<int> { 1, 2 };
foreach (var v in x)
    result.Add(v);

We try very hard to produce a collection expression that matches the style present, or looks good when no style is present. This involved effectively rewriting the entire core logic taht generates collection expressions to not use the formatting engine at all, but instead manually create everything from scratch. This adds a lot of logic, but makes the end results much higher quality.

[CyrusNajmabadi]

this helper is used by multiple fixers. SO it moved to this common location.

[CyrusNajmabadi]

same with this helper.

[CyrusNajmabadi]

this helper moved to a common location.

[CyrusNajmabadi]

this helper moved to a common location.

[CyrusNajmabadi]

this type changed a ton. It is now 3 files. The main one, with common logic. And then two siblings. One sibling is for creating object-initializers, the other for object-creation expressions.

[CyrusNajmabadi]

the collection initializer case is pretty simple. We create a list of all the expressions, then add lines breaks between the elements (we always do initializers across multiple lines).

Note: in the future we may want to copy the code that properly indents expressions so that when we move expressions into the collection initializer we do a good job with multi-line exprs.

[CyrusNajmabadi]

lots of tests got better now that we rewrote the formatting logic for collection-exprs.

[CyrusNajmabadi]

thus begins a huge swath of tests created to validate a ton of whitespace/newline trivia scenarios.

[CyrusNajmabadi]

this logic is effectively entirely new. The code heavily emphasizes trying to match the new collection-expression style against the current code the user has (or infer what would be best for certain situations). I've tried to doc it heavily as to what it's doing.

[CyrusNajmabadi]

just inlined this method.

[CyrusNajmabadi]

@akhera99 This is ready for review. THe tests should hopefully help indicate what's going on here and why so much was rewritten :)

[CyrusNajmabadi]

@akhera99 @genlu @Cosifne ptal. thanks :)

[akhera99]

Can you add a test like so

Suggested change

	if (b1)
	if (b1) // Comment

[CyrusNajmabadi]

yes. i'll add in followup pr. right now that comment will be removed.