Support for IDBFS

In order to support emulated filesystem using IDBFS, add the following to your .csproj:

<PropertyGroup>
  <WasmShellEnableIDBFS>true</WasmShellEnableIDBFS>
</PropertyGroup>

Note that this property is a shortcut to this equivalent configuration:

<ItemGroup>
    <WasmShellExtraEmccFlags Include="-lidbfs.js" />
    <WasmShellEmccExportedRuntimeMethod Include="IDBFS" />
</ItemGroup>