Added fileMustExist to DatabaseOptions
Added fileMustExist to DatabaseOptions

As per documentation: https://github.com/JoshuaWise/better-sqlite3/wiki/API

aciccarello/DefinitelyTyped,AbraaoAlves/DefinitelyTyped,benishouga/DefinitelyTyped,georgemarshall/DefinitelyTyped,jimthedev/DefinitelyTyped,benishouga/DefinitelyTyped,mcliment/DefinitelyTyped,magny/DefinitelyTyped,AgentME/DefinitelyTyped,dsebastien/DefinitelyTyped,arusakov/DefinitelyTyped,georgemarshall/DefinitelyTyped,zuzusik/DefinitelyTyped,AgentME/DefinitelyTyped,AbraaoAlves/DefinitelyTyped,zuzusik/DefinitelyTyped,benliddicott/DefinitelyTyped,AbraaoAlves/DefinitelyTyped,borisyankov/DefinitelyTyped,zuzusik/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,arusakov/DefinitelyTyped,jimthedev/DefinitelyTyped,jimthedev/DefinitelyTyped,one-pieces/DefinitelyTyped,arusakov/DefinitelyTyped,borisyankov/DefinitelyTyped,aciccarello/DefinitelyTyped,magny/DefinitelyTyped,rolandzwaga/DefinitelyTyped,benishouga/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,alexdresko/DefinitelyTyped,chrootsu/DefinitelyTyped,chrootsu/DefinitelyTyped,georgemarshall/DefinitelyTyped,alexdresko/DefinitelyTyped,rolandzwaga/DefinitelyTyped,aciccarello/DefinitelyTyped,dsebastien/DefinitelyTyped,laurentiustamate94/DefinitelyTyped