Allow gridHeight regex to match a starting decimal

[teddybradford]

Some design system libraries provide units as .75rem rather than 0.75rem. I'd like to be able to use the value directly from a library. It would be great if Plumber supported this.

I haven't thoroughly tested this, but I think something like this would work:

const unitRegExp = /^((?:\d*\.)?\d+)([a-z]{1,4})$/;