Private constant names generate warnings

[Malcolmnixon]

GDScript supports both public and private comments - with the private ones being omitted from generated GDScript documentation.

## Publicly visible constant shows up in generated documentation
const GRAVITY = 9.8

# Private constant is hidden
const _FLAGS_BUSY = 0x40

The default gdlint regular expression [A-Z][A-Z0-9]*(_[A-Z0-9]+)* does not permit leading underscores and so errors are reported:
Error: Constant name "_FLAGS_BUSY" is not valid (constant-name)

It seems like a similar single leading underscore should be permitted - _?[A-Z][A-Z0-9]*(_[A-Z0-9]+)* just as is permitted for numerous other names (functions, class-variables, etc.)

[Scony]

I think I did it on purpose to mimic pylint behavior, but I'll check once again if there's any PEP8 for that.

[Scony]

Looks like PEP8 has no strict guidelines, so let's follow Godot way - what's the Godot version you're using?

[Malcolmnixon]

We're currently targeting 4.0.3 compatibility. Thanks!

[Scony]

Ok, I'll add it to master then