fixed deprecated syntax

[filipw]

Fixes the build warnings

/Users/filipw/dev/dotnet-script/src/Dotnet.Script.Core/Internal/PreprocessorLineRewriter.cs(28,17): warning RS1034: Prefer 'IsKind' for checking syntax kinds [/Users/filipw/dev/dotnet-script/src/Dotnet.Script.Core/Dotnet.Script.Core.csproj::TargetFramework=netstandard2.0]
/Users/filipw/dev/dotnet-script/src/Dotnet.Script.Core/Internal/PreprocessorLineRewriter.cs(31,21): warning RS1034: Prefer 'IsKind' for checking syntax kinds [/Users/filipw/dev/dotnet-script/src/Dotnet.Script.Core/Dotnet.Script.Core.csproj::TargetFramework=netstandard2.0]
/Users/filipw/dev/dotnet-script/src/Dotnet.Script.Core/Internal/PreprocessorLineRewriter.cs(28,17): warning RS1034: Prefer 'IsKind' for checking syntax kinds [/Users/filipw/dev/dotnet-script/src/Dotnet.Script.Core/Dotnet.Script.Core.csproj::TargetFramework=net6.0]
/Users/filipw/dev/dotnet-script/src/Dotnet.Script.Core/Internal/PreprocessorLineRewriter.cs(31,21): warning RS1034: Prefer 'IsKind' for checking syntax kinds [/Users/filipw/dev/dotnet-script/src/Dotnet.Script.Core/Dotnet.Script.Core.csproj::TargetFramework=net6.0]
4 Warning(s)
0 Error(s)

[seesharper]

All good 👍😃