…s for indentation (#78405)

Resolves dotnet/vscode-csharp#8204

Supported platform information uses 4 spaces in its resource string `
{0} - {1}` for indentation in tool tips. However, 4 spaces at the
beginning of the line in markdown indicates a code block, breaking the
rendering of the supported platform info in LSP (thanks to @JoeRobich
for spotting that!)

Since the 4 spaces are intended to be structural indentation, we need to
use non breaking spaces (`&nbsp`) in markdown. We already handle that
generally when we see a `TextTags.Space` kind. However in this case the
4 spaces were combined with the rest of the string and therefore tagged
as `Text` instead of `Space`

The fix is to separate out the structural spacing from the rest of the
string, and tag it as such. This lets our existing handling of spaces
convert to non breaking spaces when it needs to.