src-script.js: make srcIndex into a parameter instead of a global variable

[lolbinarycat]

as far as I can tell, this is only used in createSrcSidebar, which is called in generated code like this:

var srcIndex = new Map(JSON.parse('[["test123",["",[],["test123.rs"]]]]'));
createSrcSidebar();

instead, we could have generated code like this:

let srcIndex = new Map(JSON.parse('[["test123",["",[],["test123.rs"]]]]'));
createSrcSidebar(srcIndex);