fix(radio): Custom mix script as SOURCE not parsed correctly

[philmoz]

Fixes #4230

When a custom mix script is used as a source input the value is stored as 'lua(x,y)' in the YAML file.
The ',' inside the brackets confuses the YAML parser and the value is not read back correctly.

This PR fixes the YAML parser to handle this case.

[philmoz]

This should probably be added to 2.9.2 as well as 2.10.

[boomerwing]

Thank you for the very quick action on this issue.

[pfeerick]

@raphaelcoeffic For a unit test for this, this is an example of code that was failing to be read back after a power cycle.

logicalSw: 
   0:
      func: FUNC_VEQUAL
      def: "lua(0,0),1"
      andsw: "NONE"
      delay: 0
      duration: 0

customFn: 
  0:
     swtch: "L2"
     func: PLAY_VALUE
     def: "lua(0,0),1x"

This at least visually would be retained unlike the other two...

mixData: 
 -
   weight: 100
   destCh: 0
   srcRaw: lua(0,0)
   carryTrim: 0
   mixWarn: 0
   mltpx: ADD
   offset: 0
   swtch: "NONE"
   flightModes: 000000000
   delayUp: 0
   delayDown: 0
   speedUp: 0
   speedDown: 0
   name: ""