add support for basic findOne with ID

[intellix]

following on from #147

I decided to see if Claude could help me fix the test, since this work is so valuable to us and it seems to have it all passing.

Supports dataloading for cases such as:

User.findOne({ where: { id: 1 } })
User.findOne({ where: { id: 2 } })
// → SELECT ... FROM users WHERE id IN (1, 2)

User.findOne({ where: { id: 1, status: 'active' } })
User.findOne({ where: { id: 3, status: 'active' } })
// → SELECT ... FROM users WHERE id IN (1, 3) AND status = 'active'

User.findOne({ where: { id: 2, status: 'inactive' } })
// → SELECT ... FROM users WHERE id IN (2) AND status = 'inactive'  (separate loader)

User.findOne({ where: { id: 1, [Op.and]: [{ deletedAt: null }] } })
User.findOne({ where: { id: 2, [Op.and]: [{ deletedAt: null }] } })
// → SELECT ... FROM users WHERE id IN (1, 2) AND deletedAt IS NULL

User.findOne({ where: { id: 1, enabled: true }, order: [['id', 'DESC']] })
User.findOne({ where: { id: 2, enabled: true }, order: [['id', 'DESC']] })
// → SELECT ... FROM users WHERE id IN (1, 2) AND enabled = true ORDER BY id DESC

User.findOne({ where: { id: 3, enabled: true }, order: [['id', 'ASC']] })
// → SELECT ... FROM users WHERE id IN (3) AND enabled = true ORDER BY id ASC  (separate)

Also, it adds a new BATCH_BY_ATTRIBUTE to easily batch by another attribute like a foreignKey:

import { EXPECTED_OPTIONS_KEY, BATCH_BY_ATTRIBUTE } from 'dataloader-sequelize';

Box.findOne({
  [EXPECTED_OPTIONS_KEY]: context.dataloaderContext,
  [BATCH_BY_ATTRIBUTE]: 'ancestorBoxId',
  where: { ancestorBoxId: 1, enabled: true },
});
Box.findOne({
  [EXPECTED_OPTIONS_KEY]: context.dataloaderContext,
  [BATCH_BY_ATTRIBUTE]: 'ancestorBoxId',
  where: { ancestorBoxId: 2, enabled: true },
});
// → SELECT * FROM boxes WHERE ancestorBoxId IN (1, 2) AND enabled = true

[intellix]

let me know what you think @mickhansen - if there's anything I need to defend or test against and if you like the BATCH_BY_ATTRIBUTE stuff