fix: Fix sniper printing of comments when changing modifiers

[slarse]

Fix #3697

This is heavily WIP, I don't have a fix yet. The problems are thoroughly described in #3697

This PR introduces a fix for the problem of disappearing whitespace between comments and type members with modified modifier lists. For example, given the following commented type member:

// this is a type member
private static int number = 2;

Adding a final modifier to the modifier list would result in the following print-out:

// this is a type memberprivate static int number = 2;

The source fragments will look something like so:

"// this is a type member"
["\n\t", "private", "static"]
...

Where the comment is a normal ElementSourceFragment and the list is a CollectionSourceFragment. The reason the whitespace isn't printed is that the it becomes part of the CollectionSourceFragment. The problem with this is that the modifier list is only entered when the first modifier is encountered, in this case private, and then the preceeding whitespace is printed. But as the CollectionSourceFragment hasn't been entered, the only preceeding whitespace is the comment (which is treated as whitespace).

The fix this PR does is to make the whitespace its own separate source fragment, like so.

"// this is a type member"
"\n\t"
["private", "static"]

This fix is only applied when the whitespace preceding a CollectionSourceFragment is the suffix whitespace of a comment. This is the only case where I've found it to be necessary.

Ps. This explanation may seem overly long, but I'm documenting this mostly for myself if this causes a bug tomorrow, and I've forgotten all about it.

[slarse]

I've been working on this for half a workday now. I'm not sure I can fix this reasonably, there are simply too many corner cases with the way the sniper printer currently works.

[slarse]

My new attempts at fixing this seem to have worked. It's really two entirely different problems. I'll write up an explanation tomorrow.

Ps. Not ready to merge, need to tidy up.

[slarse]

This is ready for review, see the initial comment for an explanation.

[slarse]

There is an issue related to this where removing the final element in a modifier will cause any preceding comment to disappear, but it's a separate implementation issue and so I'm treating it separately (see #3732).

[monperrus]

LGTM. Will merge. Thanks!

[monperrus]

The sniper mode is ever better! Thanks @slarse