Fields: Have each type independently serialize

[henrygab]

Originally posted by @tobi01001 in https://github.com/jasoncoon/esp8266-fastled-webserver/pull/216#discussion_r757550079

In convertToJson(), the request is to have each unique Field_t serialization handled in the switch statement directly, rather than code multiple if statements that each run code that is common to a random subset of the types.

This will help make the code easier to understand, which reduces bugs.

On platforms that use F-Strings (ESP8266, AVR), there will be a small cost that cannot be optimized, due to the optimizer's inability to consolidate F-strings. On platforms with flat memory model (ESP32, most ARM based mcu), it is expected that the compiler will consolidate common code paths more efficiently than human hand optimization.