Initiate _tikzUseTectonic in init

[Ellipse0934]

Changing init from __init__() = __init__svg() to

function __init__()
    __init__svg()
    try
        success(`$(tikzCommand()) -v`)
    catch
        global _tikzUseTectonic
        _tikzUseTectonic = true
    end
end

Automatically selects tectonic upon loading. We can additionally consider adding an environment variable with which users can specify which backend they wish to use.

Happy to file a PR, any thoughts ?

[mykelk]

Sure, you can file a PR!