Placeholder names with uppercase letters

[burntcustard]

placeholder-name-format:
    - 1
    - convention: hyphenatedbem

Doesn't allow for uppercase characters, so for example:

%u-translateX--50pc;

Comes up with a warning!

I'd like to allow these, because some CSS values that we're setting with our placeholders traditionally get written with uppercase letters, like transform: translateX()

So I think we should

placeholder-name-format:
    - 1
    - convention: ^[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*(?:__[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*)?(?:--[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*)*(?:\\[.+\\])?$
    - convention-explanation: 'hypenatedbem uppercase & multiple modifiers allowed'