[nanoutils] cannot use create-index for short folders

[Beraliv]

Real examples:

• __
• F
• o
• T

These methods haven't been included before, you can check it for https://www.npmjs.com/package/nanoutils/v/0.1.1 and previous versions (unfortunately 😢)

Solution: you can start file from _ and one symbol is required to make folders and files safe (not 2 as before)

[Beraliv]

ping @gajus
what do you think?

[gajus]

I think these are horrible variable names, but as long as they are valid variable names, this library can support them.