Fix buffer index crashes

[jojorne]

Notes

• Position.Line doesn't take into account trimmed lines.
• It appears that the axaml doesn't care about the line feed.

[jojorne]

But I think something like this would be better, especially if you want to reuse GetLine.

namespace AvaloniaLanguageServer.Services;

public sealed class Buffer
{
    public string GetTextTillLine(Position position)
    {
        var text = string.Join("\n", _lines[..position.Line]);
        var line = GetLine(position)[..position.Character];
        return $"{text}\n{line}";
    }

    public string GetLine(Position position)
    {
        return _lines[position.Line];
    }

    public string GetText()
    {
        return string.Join("\n", _lines);
    }

    public Buffer(string text)
    {
        _lines = text.Split(separator, StringSplitOptions.None);
    }
    
    private readonly string[] _lines;
    private static readonly string[] separator = { "\n", "\r\n" };
}

[microhobby]

Hey @prashantvc this should fix #110 and #113

[microhobby]

LGTM, thanks for the contribution @jojorne !

[jojorne]

Any idea when the vscode extension will be updated? 🤔
It's just that this project seems abandoned... 😅