Add config option to enable location tracking in prisma-ast

[MrLeebo]

Makes CST node locations available in the prisma-ast schema. This information isn't presented via the builder API, but you can access it by calling const schema = getSchema(source) directly.

Location statistics are:

startLine: number;
startColumn: number;
startOffset: number;
endLine: number;
endColumn: number;
endOffset: number;

Closes #21