Scan code for save() (and other issues) before upload

[gfwilliams]

It seems common to put save() at the end of code in the editor. This works (but is not recommended) when you upload to RAM, but if you upload to flash it's a disaster, causing a large flash write (and reset) every time the device boots.

We should detect this (and maybe other issues) before upload and warn...

[gfwilliams]

Should also scan for reset = and other assignments that might overwrite built-in functions.

Maybe we should just use eslint in codemirror and show the errors as they are being written?