Replace usage of StringReader.ReadLine() with something else

[rjmholt]

We use StringReader.ReadLine() to read in a file for the first time here:

PowerShellEditorServices/src/PowerShellEditorServices/Workspace/ScriptFile.cs

Lines 224 to 237 in 042f2e0

	using (var reader = new StringReader(text))
	{
	// 50 is a rough guess for typical average line length, this saves some list
	// resizes in the common case and does not hurt meaningfully if we're wrong.
	var list = new List<string>(text.Length / 50);
	string line;
	while ((line = reader.ReadLine()) != null)
	{
	list.Add(line);
	}
	return list;
	}

However, this has the nasty habit of dropping newlines at the ends of files. See https://github.com/dotnet/corefx/issues/32990.

This causes off-by-one errors in all these files for us. But since this behaviour is in both .NET Framework and Core, it's probably permanent. So we just need to use something else.

In the above corefx issue, I summarised different string split methods, and think we should either use String.Split() or do it ourselves, depending on what we think is best suited to our needs.