use proper escaping, not \uXXXX

reject \uXXXX

convert \u{X} to \uXXXX - even for supplemental

escape marker matches

It turns out that we need to use char literals instead of escapes so that normalization works properly. e<marker>\\u0300\\u0320 needs to actually be e<marker>U+0300U+0320 so that the normalization can apply - it won't apply to the escaped chars! So probably make a list of which chars are safe to unescape (skipping parens, brackets etc).

handle \u{...} in reorders