Add format_on_save toggle for javascript

[mekanix]

PR Prelude

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Why this change is necessary and useful?

[Please explain in detail why the changes in this PR are needed.]

I used python.vim as an example how format_on_save is handled and added it to javascript.

[wsdjeg]

in javascript layer you can use following instead:

 let s:format_on_save = get(a:var,
        \ 'format_on_save',
        \ s:format_on_save)

format-on-save is just for backword compatible in python layer.

[mekanix]

It's amended.

[wsdjeg]

Thanks, and can you update the lang#javascript layer doc.

[mekanix]

Done.

[wsdjeg]

Merged, thanks.