feat: import pg-aggregates plugin with Constructive adaptations

Import @graphile/pg-aggregates v5 branch via git subtree, then adapt for
Constructive's codebase conventions and custom connection-filter fork.

Changes from upstream:
- Build system: yarn/tsc → pnpm/makage with CJS+ESM outputs
- Imports: Replace subpath exports (postgraphile/grafast, graphile-build-pg/@dataplan/pg)
  with direct package imports for moduleResolution: node compatibility
- FilterRelationalAggregatesPlugin: Read from $where._manyRelation (our
  ConnectionFilterBackwardRelationsPlugin) instead of upstream's
  $where.extensions.pgFilterRelation
- Plugin ordering: before ConnectionFilterArgPlugin (our fork's name)
- Field naming: Aggregate sub-filter field renamed from 'filter' to 'where'
- Version: Inline const version instead of require('../package.json')
- Remove .js extensions from local imports
- Add PgConditionExtensions augmentation for pgFilterAttribute
- Add graphile-connection-filter and grafast as peer dependencies
- Wire PgAggregatesPreset into ConstructivePreset in graphile-settings
- Remove upstream test files, eslint config, yarn.lock, graphile.config.js

Provides: aggregates field on connections (sum, avg, min, max, stddev,
variance, distinctCount), groupedAggregates with groupBy + having,
orderBy relational aggregates, filter by relational aggregates.
Per-table opt-out via @behavior -aggregates smart tag.

Author: pyramation