Include guard refactors

[nbfalcon]

• Add an include guard
• Remove an include guard
• Add #pragma once
• Remove #pragma once
• Toggle between include guards/#pragma once
• All of the above exposed in the menu, context sensitive (i.e. no "remove include guards" if there are none)
• Skips comments at the start and end of the buffer (i.e. copyright boilerplates, "LocalWords: ")
• A lot of customization options (custom include guard symbol generators, text after #endif, spaces after #, ...)
• Handles lots of tricky edge cases:

#ifndef Q
#define Q

in\
t main() {}

#endif /* Q */```

->

```c++
#ifndef Q
#define Q

in\
t main() {}

#endif /* Q */

Fixes #62.

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