Add option to use `retval` for boolean return values

[LordFlashmeow]

Feature

Implements #201. If enabled (and when boolReturnsTrueFalse enabled), replaces the returns keyword with retval for bool functions.

I chose to make it controlled by a new setting, rather than using the boolReturnsTrueFalse setting because some users might prefer the existing behavior (return for true/false).

\retval Doxygen source

Sample behavior:

/**
 * @brief
 * 
 * @retval true
 * @retval false
*/
bool get() {}

• [x] Description of what's added
• [ ] ~Gif of your feature if appropriate~
• [ ] ~Added gif to README~
• [x] Added unit test