Add compiler.AllowUnsafe = true; to allow building unsafe code

[rapidgiga]

Some functions return pointers:

However this results in the following build failure:

[DotX64Dbg] Rebuilding plugin 'RapidString'...
[DotX64Dbg] Build failed
C:\Users\rapid\Documents\x64dbg\release\dotplugins\RapidString\plugin.cs(4,29): error CS0227: Unsafe code may only appear if compiling with /unsafe
C:\Users\rapid\Documents\x64dbg\release\dotplugins\RapidString\plugin.cs(9,23): error CS0012: The type 'IsSignUnspecifiedByte' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime.CompilerServices.VisualC, Version=6.0.0.0, Culture=neutral, PublicKeyToken=[b03f5f7f11d50a3a](x64dbg://localhost/address64#b03f5f7f11d50a3a)'.
[DotX64Dbg] Build failed

This can be resolved by adding compiler.AllowUnsafe = true; to this file:

DotX64Dbg/src/Dotx64Managed/Compiler.cs

Line 140 in 7670610

var options = new CSharpCompilationOptions(forScripting ? OutputKind.ConsoleApplication : OutputKind.DynamicallyLinkedLibrary)