Remove decimal.js in favor of Prisma.Decimal

[lakardion]

Prisma already uses decimal.js underneath. So we can remove the library from our main dependencies, and let us use Prisma's internal decimal.js.

[lakardion]

Turns out that Prisma.Decimal does not work on client side. It can only be used on the server

(may be missing something here, but this is the practical response I got when attempted to use it in the client)