Add default fileNesting contributions

## Describe the solution you would like
Per the docs, extensions can contribute default Explorer fileNestingPatterns: https://code.visualstudio.com/updates/v1_67#_explorer-file-nesting

We should provide a set of defaults as follows:
```
"explorer.fileNesting.patterns": {
"*.cs": "${capture}.designer.cs,${capture}.g.cs,${capture}.generated.cs",
"*.csproj": "${capture}.csproj.user",
"*.json": "${capture}.Development.json",
"*.cshtml": "${capture}.cshtml.cs,${capture}.cshtml.css",
"*.razor": "${capture}.razor.cs,${capture}.razor.css",
"*.css": "${capture}.css.map"
}
```

## Applicable Scenarios
Default VS Code explorer view

## Additional context
Ionide has done this in https://github.com/ionide/ionide-vscode-fsharp/issues/1724