[Rules] Add string functions unescape, escape, parse and json

[tonhuisman]

Inspired by #5597 and some Forum requests here and here, and the long-standing TODO for proper JSON parsing.

Features:

• String function {unescape:<string-to-unescape>} (remove \ escape-prefixes from % [ ] { } ( ) and :).
• String function {escape:<string-to-escape>} (insert the \ prefix to % [ ] { } ( ) and :).
• String function {parse:<param>:[<separator>]:<string-to-parse-from>} fetch the 1-based parameter from the string, using the separator (default when empty: ,).
• String function {json:<json-path>:[<asJson>]:<json-string>} retrieve a value from a JSON (escaped) string, using the often used name.sub.array[n].attr notation. Returns numeric, string, bool, array(as csv) and object(as name,value csv) values. When asJson is 1, returns a JSON-compatible value (quoted strings, bool: true/false, []-wrapped arrays and {}-wrapped objects.
• The extracted JSON parser is based on the code from HTTPResonseParser.cpp, adding support for bool and object values, and also nesting (recursive). Has a feature-flag FEATURE_JSON__PARSER that auto-enables if P037 or the HTTP_RESPONSE_PARSER are included.
• [HTTPResponseParser] Use the extended JSON parser.
• [P037] Use the extended JSON parser instead of just the simple name.attr parser it had.
• [Docs] Added for the new string functions.
• [EasyCode] Keywords added.

NB: These features are only available in ESP32 builds, as it won't fit in the current set of ESP8266 builds (see our policy here). It can be enabled for a Custom build, using the feature flags FEATURE_EXTENDED_STRING_FUNCTIONS and FEATURE_JSON_PARSE (enable both to get the JSON-parse feature).

TODO:

• Testing
• Extend docs where needed