Create a file naming convention

[milkbump]

Looking through V's source files, the naming convention doesn't seem entirely consistent. While I've mostly seen snake_case, I've also seen kebab-case. It seems to be a point of contention in Go. So now would be a nice time to use and document a convention.

Notable comment from the linked Go issue above:

A short new section on CodeReviewComments is more than sufficient.

Example:

There is a de facto standard for V source file names: all lowercase with underscore separation when necessary, i.e. snake case.

Thoughts?

Edit: most of the snake case violations happen in v/cmd/tools/

[spytheman]

They are named like that, since they match the name of the command that they implement:
v test-fixed => cmd/tools/vtest-fixed.v
v setup-freetype.v => cmd/tools/vsetup-freetype.v
... and so on.

I guess it was just easier to type the name, without using shift at the time. It is easy to rename them, if we want to enforce a naming convention using _ instead of - for the file names.

@medvednikov what do you think?