Add constants and MERCINFO_GID to *getmercinfo script command

[AnnieRuru]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR will be closed if the above-mentioned criteria are not fulfilled.

Issues addressed

while fixing #2395 and I need a script command to return mercenary GID,
so I copy existing code from rAthena

Changes Proposed

Add constants and MERCINFO_GID to *getmercinfo script command

Tested with

prontera,155,185,5	script	kjdshfksdf	1_F_MARIA,{
	getitem Bow_Mercenary_Scroll1, 1;
	
	for ( .@i = 0; .@i <= 8; ++.@i )
		mes getmercinfo(.@i) +"";
	
	mes rid2name(getmercinfo(MERCINFO_GID));
	mes getunitname(getmercinfo(MERCINFO_GID)); // return nullpo
	close;
}

Affected Branches

• Master

Known Issues and TODO List

none

[HerculesWSAPI]

This change is 

[AnnieRuru]

fix line break coding style in switch cases