add support of script_startup together with script

[stepansnigirev]

Relevant discussion here:
https://forum.littlevgl.com/t/examples-for-lvgl-in-micropython/1190/26

With this pull request an optional parameter script_startup can be passed alongside with the script to lvgl_editor.html, for example:

https://stepansnigirev.github.io/sim/lvgl_editor.html?script_startup=https://gist.githubusercontent.com/stepansnigirev/d360caaca643a17989b1b41ceef417d1/raw&script=https://raw.githubusercontent.com/stepansnigirev/lv_examples/rework/src/lv_ex_chart/lv_ex_chart_1.py

I renamed a parameter to script_startup for consistency with script and script_direct.

The processScriptArg function is changed to be extendable to more scripts in the future if we want to. The first argument of the function can be an url string, then it works as before, or it can be an array of objects, then all of them will be loaded and combined to the editor window. If comment is present in the object it will add the corresponding comment before the script.

[embeddedt]

FYI: As far as I know external_link does nothing anymore, so all of the lines referring to it (and prev_external_link) can be removed.