regexp/syntax: recognize Unicode category aliases

The regexp package in Go can match Unicode characters using the \p{…} syntax. Before this change, it only understood the short two‑letter codes like \p{L} for letters. It did not recognize longer names such as \p{Letter} or the special groups \p{Any}, \p{Assigned}, and \p{ASCII}. The new change adds support for those longer names and for the pseudo‑categories. It also makes the names case‑insensitive, so \p{ascii} works the same as \p{ASCII}. With this update, patterns that use the full Unicode names will work correctly, making the package more aligned with the Unicode standard and easier to use in real‑world text processing.