Add MSVC debugger visualization file

[cmakshin]

This pull request contains a file which can be used by the Visual Studio debugger for "pretty-printing" tsl::sparse_* containers.

[Tessil]

Hello,

I made a couple of corrections similar to the natvis of the ordered-map project.

I was also wondering if the visualization for tsl::sparse_map that shows the key in the name column was necessary? As it add extra code to maintain, what would be the advantage of having this view in addition to the first one?

[cmakshin]

Hello.

Showing the key in the name column was done for two reasons:

1. I find it more convenient for inspecting the container's contents;
2. that's how std::map visualizer works [by default]. std::unordered_map visualizer also shows the key in the name column, but, for some reason, the whole key-value pair in the value column.

[Tessil]

Effectively.

I have to check a bit as the other natvis files you contributed just use a simple index (which is preferable for tsl::ordered_map). I have to see if I can add the key in the name of the column for the other projects in tsl as I would like to keep some consistency across the projects.

[Tessil]

I think it should be better to keep the (key, value) in the column index to mimic more closely what Microsoft did with std::unordered_map. If they change it in the future, I'll see to change it here too.

[Tessil]

I merged the changes, thank you for your contribution.