Added solution and project files in VS2019 format, targeting .NET Standard 2.0 (netstandard2.0) and .NET Framework 4.5.2 (net452)

Author: apdevelop

.gitignore

@@ -1,5 +1,7 @@
 ## Ignore build results and temporary files.
 
+.vs
+
 Backup*
 _UpgradeReport_Files
 bin

WebSocketSharp.sln

@@ -0,0 +1,25 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 16
+VisualStudioVersion = 16.0.31005.135
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "WebSocketSharp", "websocket-sharp\WebSocketSharp.csproj", "{9819518E-D926-48CA-8636-AAECFC2CB654}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{9819518E-D926-48CA-8636-AAECFC2CB654}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{9819518E-D926-48CA-8636-AAECFC2CB654}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{9819518E-D926-48CA-8636-AAECFC2CB654}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{9819518E-D926-48CA-8636-AAECFC2CB654}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {B178666C-9436-4B3C-AD94-83CA3E4F751E}
+	EndGlobalSection
+EndGlobal

websocket-sharp/WebSocketSharp.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+	<PropertyGroup>
+		<TargetFrameworks>netstandard2.0;net452</TargetFrameworks>
+		<AssemblyName>WebSocketSharp</AssemblyName>
+		<RootNamespace>WebSocketSharp</RootNamespace>
+		<GenerateAssemblyInfo>false</GenerateAssemblyInfo>
+		<GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+		<Deterministic>false</Deterministic>
+	</PropertyGroup>
+
+	<PropertyGroup>
+		<GenerateDocumentationFile>true</GenerateDocumentationFile>
+	</PropertyGroup>
+
+	<ItemGroup>
+		<Compile Remove="doc\**" />
+		<EmbeddedResource Remove="doc\**" />
+		<None Remove="doc\**" />
+	</ItemGroup>
+
+	<ItemGroup>
+		<None Remove="websocket-sharp.snk" />
+	</ItemGroup>
+
+</Project>