Upgrade to .NET 6.0

This adds csproj-files for building using .NET 6.0 and adds assembly
attributes required for loa...

Otherwise we might prematurely free Externals, since the object with the
finalizer has nothing t...

Cleanup and update to V8 5.5

This ensures that Persistent handles are Reset when destructed, which
they didn't before, leadin...

Otherwise this can lead to crashes.

They don't work very well but may help next time.

Requires building for OSX on OSX, for Android...

So the user of the C# lib doesn't have to use a StringBuilder.

Plus some callback tweaks.

And some minor fixes

Etc

This should remove some layers of indirection created by Swig, and
generally cleans up the inter...

Instead of just the name. For memory the C# memory handling to work out,
it can only be gotten o...

The void* constructor installs a new finalizer, but at this point we can
assume that it has alre...

For holding on to pointers from the host language

This fixes a bug where callbacks weren't properly cleaned-up after.

And re-use isolates when re...

ThrowException should be used when there are exceptions inside
Callbacks.

And explicitly convert from C# strings to UTF8-encoded byte arrays

And add Equals and GetHashCode functions that work on the native
reference to the C# interface.

For binary compatibility

This causes configuration problems on Windows

to avoid linker problems

For easier interoperability

when that's not valid

and explicitly delete copy constructors/operators that MSVC otherwise
complains about when using...

This fixes multithreading problems

To ease memory management