Change solution file parsing to report the structure of folders/projects, rather than returning projects as a flat list

[Numpsy]

refs ionide/ionide-vscode-fsharp#2107

An attempt at making it return the projects inside their solution folders, rather than along side them, by making it work more like v0.68.0 did (I created the new unit test by writing the test in the 0.68.0 version on the assumption that's returning the strucuture that Ionide expected, then moving the test to the latest source and changing the code to match.)

Possibly this still needs more tests adding (maybe both a sln and a slnx file to make sure they behave the same, and maybe multipe layers of nested subfolders rather than just one)

I also haven't tested this within Ionide yet - just the unit tests so far.

[Numpsy]

When testing I noticed that some solution items are being returned twice here, which didn't seem right (note the duplicated guids in the results, with different Kinds)

Looks to be because 'SolutionItems' contains all the folders and so the existing logic will include any root level folders as 'Unknown' as well as including them as folders

[Numpsy]

Updated based on comments in ionide/ionide-vscode-fsharp#2109 about returning the solution folders before the solution level projects.

Still needs a bit more testing with the interaction between solution filters and folders (what should happen with folders that have all their contents filtered out etc)

[Numpsy]

Ionide currently displays the proj-info solution like this

and after this change that becomes

Visual Studio 2022 shows this for comparison:

[Numpsy]

Still needs a bit more testing with the interaction between solution filters and folders (what should happen with folders that have all their contents filtered out etc)

It looks like if you have

• A solution with nested solution folders, with projects at each level
• A solution filter that doesn't include the projects at the bottom of the folder tree

Then Visual Studio doesn't display the empty folders, but Ionide does,
Hoever, maybe this is a seperate bug, and I'm not sure if proj-info should filter out such folders here, or if something higher up should do it?

[Numpsy]

Still needs a bit more testing with the interaction between solution filters and folders (what should happen with folders that have all their contents filtered out etc)

It looks like if you have

* A solution with nested solution folders, with projects at each level

* A solution filter that doesn't include the projects at the bottom of the folder tree

Then Visual Studio doesn't display the empty folders, but Ionide does, Hoever, maybe this is a seperate bug, and I'm not sure if proj-info should filter out such folders here, or if something higher up should do it?

Opened #242 just for the filtering question, to leave this one just for the folder structure changes