lark: support \xHH escapes in string literals (#354)

Fixes the `\xHH` case of #54. Lark string literals are decoded through
`serde_json`, which rejects `\xHH`, so this accepts `\xHH` in the string
lexer and rewrites it to the equivalent `\u00HH` before decoding, with a
test. Broader Python-Lark escape parity, notably `\UXXXXXXXX`, can
follow separately.