Rename File.real_path to .realpath

[straight-shoota]

The spelling is almost everywhere without an underscore:

• CLI util: realpath
• LibC function: realpath
• no other method on File with similar name uses an underscore: basename, dirname, tempfile etc. (expand_path is a combination of verb and subject)
• even in Ruby (which the file API is based on): File.realpath
• and dozens of other file APIs: realpath

So it's pretty clear that the spelling real_path is an odd incidence. I regularly misspell it.

I'd like to rename the method to File.realpath because it just makes most sense.

[asterite]

Yeah, I think we tried to "fix" this compared to Ruby, but Ruby was right 😅