Support regex for mapping field names

[jmattheis]

When you have fields that are flattened and then prefixed with the nested field name, it would be useful when goverter would support regex.

This example is pretty verbose:

// goverter:map DBObject.ID ID
// goverter:map DBObject.CreatedAt CreatedAt
// goverter:map DBObject.UpdatedAt UpdatedAt
// goverter:map VictoryConditions.NumCriteriaRequired VictoryConditionsNumCriteriaRequired
// goverter:map VictoryConditions.YearsPassed VictoryConditionsYearsPassed
// goverter:map VictoryConditions.OwnPlanets VictoryConditionsOwnPlanets
// goverter:map VictoryConditions.AttainTechLevel VictoryConditionsAttainTechLevel
// goverter:map VictoryConditions.AttainTechLevelNumFields VictoryConditionsAttainTechLevelNumFields
// goverter:map VictoryConditions.ExceedsScore VictoryConditionsExceedsScore
// goverter:map VictoryConditions.ExceedsSecondPlaceScore VictoryConditionsExceedsSecondPlaceScore
// goverter:map VictoryConditions.ProductionCapacity VictoryConditionsProductionCapacity
// goverter:map VictoryConditions.OwnCapitalShips VictoryConditionsOwnCapitalShips
// goverter:map VictoryConditions.HighestScoreAfterYears VictoryConditionsHighestScoreAfterYears
// goverter:map VictoryConditions.Conditions VictoryConditionsConditions
// goverter:map Area.X AreaX
// goverter:map Area.Y AreaY
ConvertGameGame(source *cs.Game) *Game

but with regex this is fairly easy to describe:

// goverter:map:regex DBObject.(\w+) $1
// goverter:map:regex VictoryConditions.(\w+) VictoryConditions$1
// goverter:map:regex Area.(\w+) Area$1
ConvertGameGame(source *cs.Game) *Game

Example from: https://github.com/sirgwain/craig-stars/blob/e3683743518428ee08bf8e63bbf1483f3d1e517f/db/converter.go#L140

Please 👍 this issue if you like this functionality. If you have a specific use-case in mind, feel free to comment it.