feat(variables): custom builder function to override variable generation

[BasitAli]

This allows specifying custom builder methods for variable definition to cater requirements such as #22 and #65.

import * as gql from 'gql-query-builder'

function customVarBuilder(key: string, value: BuilderVariableOptions) {
  const name = value.name ?? key;
  return `where: {${name}_eq: $${name}}`;
}

const query = gql.query({
  operation: 'userByEmail',
  variables: {
    email: { value: '[email protected]', required: true, builder: customVarBuilder}
  },
  fields: ['userId', 'email']
})

console.log(query)

// Output
query ($email: String!) {
  userByEmail (where: {email_eq: $email}) {
    userId, email
  }
}

// Variables
{
  email: "[email protected]"
}