Fallback gracefully when running on runtimes that only support the Invariant culture

[CyrusNajmabadi]

Fixes microsoft/vscode-dotnettools#386.

[CyrusNajmabadi]

@dotnet/roslyn-ide @davidwengier @dibarbet for this simple resiliency pr on some stripped down distros.

[CyrusNajmabadi]

@genlu @dibarbet ptal.

[jasonmalinowski]

Does having this replaced in break any of the other code expecting this to be en-US? For example, the check here:

if (culture.ThreeLetterWindowsLanguageName.Equals(EnUSCultureInfo.ThreeLetterWindowsLanguageName))

That doesn't do anything strange now?

[dibarbet]

From a basic test it at least doesn't throw

Console.WriteLine(CultureInfo.InvariantCulture.ThreeLetterWindowsLanguageName);
Console.WriteLine(CultureInfo.CurrentUICulture.ThreeLetterWindowsLanguageName.Equals(CultureInfo.InvariantCulture.ThreeLetterWindowsLanguageName));

writes

IVL
False

Docs also say this (though not 100% the same)

It can be used in almost any method in the [System.Globalization](https://learn.microsoft.com/en-us/dotnet/api/system.globalization?view=net-7.0) namespace that requires a culture. The objects returned by properties such as [CompareInfo](https://learn.microsoft.com/en-us/dotnet/api/system.globalization.cultureinfo.compareinfo?view=net-7.0), [DateTimeFormat](https://learn.microsoft.com/en-us/dotnet/api/system.globalization.cultureinfo.datetimeformat?view=net-7.0), and [NumberFormat](https://learn.microsoft.com/en-us/dotnet/api/system.globalization.cultureinfo.numberformat?view=net-7.0) also reflect the string comparison and formatting conventions of the invariant culture.

[CyrusNajmabadi]

That doesn't do anything strange now?

It may. But i don't believe that's somethign we should care/invest in. This gets us out of server crashing, which is the pri0 issue. Having slightly incorrect sorting/filtering in pattern matching on this niche platform is pri4/sev4 if it happens.