Support square bracket notation in paths

[thecrypticace]

Fixes #6436

Add support for square bracket notation in theme paths.

You can again do things like:

theme('colors[yellow].100')
theme('colors[red][100]')
theme('spacing[1.5]')

This uses a small parser to pick out the individual parts and, for invalid paths, provide messages about where the error is in the path itself. For example if there are mismatched or unclosed brackets: spacing[1.5]]