Request.from_curl() with $-prefixed string literals

[wRAR]

Chrome (and probably other things) sometimes generate curl commands with a $-prefixed data string, probably when it's easier to represent the string in that way or when it includes non-ASCII characters, e.g. the DiscoverQueryRendererQuery XHR on https://500px.com/popular is copied as

curl 'https://api.500px.com/graphql' \
<headers omitted>
  --data-raw $'{"operationName":"DiscoverQueryRendererQuery",<omitted> "query":"query DiscoverQueryRendererQuery($filters: [PhotoDiscoverSearchFilter\u0021], <the rest omitted>' \
  --compressed

, most likely because of \u0021 in this payload.

scrapy.utils.curl.curl_to_request_kwargs() isn't smart enough to understand this kind of shell escaping, so it puts $ into the request body which is incorrect. Ideally we should support this, though I don't know if there are existing libraries to unescape this.