Renaming variables

Some variables have too generic names that do not indicate their role.
Renaming them in accordance with their role and the constraints below will make the project easier to understand and debug.

## Constraints
1. You should understand their role based on their name (see Variables description in CONTRIBUTING)
2. All variables must be prefixed by `JO_JOIN_`
3. All CAPS, exception if the variable is linked to a trigger, like `JO_Myself_LeaveParty` which is similar to `LeaveParty(Myself)`
4. The name must be <= 32 characters long, ideally <= 24.
5. ?

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- [ ] Add Constraints to CONTRIBUTING
- [ ] Re-read the name variables and explain it, while respecting the constraints
- [ ] After validation by the maintainer
- [ ] Upgrade CONTRIBUTING file
- [ ] Use them in the code

Bad examples:
- `JO_JOINI`
- `JO_JOIN_JOIN`

Various proposals:
~`JO_JOIN_AREA` → `JO_JOIN_IS_MASTER_AREA`~
~`JO_JOIN_SWITCHING` → `JO_JOIN_SWITCHING_PARTY`~
~`JO_JOIN_LOADING` → `JO_JOIN_LOADING_SWITCH`~
~`JO_JOIN_CLEAR` → `JO_JOIN_SET_JOINX`~
`JO_JOIN_IS_HIDDEN` → `JO_JOIN_IS_HIDDEN_BY_CUT`
`JO_JOIN_Move_Around` → `JO_JOIN_DLG_OPTION`
`JO_JOIN_Move_Done` → `JO_JOIN_DLG_OPTION_DONE`
`JO_JOIN_SWITCH_WITH_ME` → `JO_JOIN_SWITCH_NEXT_SLOT`
~`JO_JOIN_Wannabe` → `JO_JOIN_WANNABE_TRAVELER`~
~`JO_JOIN_IS_Wannabe` → `JO_JOIN_WANNABE_INIT`~
~`JO_JOIN_READY_TO_SWITCH` → `JO_JOIN_PARTY_SWITCH_STATE`~
~`JO_JOIN_NEVER_SWITCH` → `JO_JOIN_NO_PARTY_SWITCH`~
`JO_JOIN_FORCE_SWITCH` → `JO_JOIN_FORCE_QUICK_SWITCH`
`JO_JOIN_NEVER_JOIN` → `JO_JOIN_NEVER_QUICK_SWITCH`
`JO_JOIN_Fill_Party` → `JO_JOIN_READY_TO_FILL`