Spring Revise handling of URI variable syntax in WhatWG URL parser

The new WhatWG URL parser enters a special URI_TEMPLATE state on open {, and remains there until }, effectively suspending checks for general delimiters at URI component boundaries. This is in contrast to the regular expressions used before, which did not do handle curly braces specially, except in the port, but otherwise parsed along general delimiters.

We should revise how the new parser handles URI variable syntax to match closer what we did before, prioritizing safer parsing and better aligning with how user agents might parse the same, in line with the goals for the parser.