fix: constants, parserCache, Pool and Pool Cluster typings

[wellwelwel]

It's expected that this PR will solve:

• Issues
  • Module "mysql2" has no exported member "Types" #1938
  • Typescript: Pool Cluster Typing Mismatch #1991
  • TypeScript: missing, wrong and refactoring types #2072
• Milestones
  • TypeScript: fixes, tests and refactoring
• Probably also these PRs:
  • Add correct Types for PromisePoolCluster #2035
  • fix: update pool cluster of to use pool namespace #2036

Naturally, addressing all remaining known type-related issues related to version ^3.x 🤹🏻‍♀️

---

Constants

Reviewed ✔️

• JS (required to fix the types)
  • Add missing constants to promise.js:
    • Types
    • Charsets
    • CharsetToEncoding
• TS
  • Create typings and tests for:
    • Types
    • Charsets
    • CharsetToEncoding

---

Parsers

Reviewed ✔️

• JS (required to fix the types)
  • Add missing parserCache methods to promise.js:
    • setMaxParserCache
    • clearParserCache
• TS
  • Create typings and tests for:
    • setMaxParserCache
    • clearParserCache

---

Pool

Reviewed ✔️

• TS
  • Add URI overload and testing to createPool

---

Pool Cluster

Fix mysql2 first, then create mysql2/promise typings ✔️
Reviewed ✔️

• TS
  • Add missing Connection URI overload, like poolCluster.add('conn', 'uri')
  • Add missing config option uri in add method
  • end, of and add methods can't be in a PoolCluster connection
  • of type can't be a PoolCluster
  • Create Pool Cluster typings for promise.d.ts
  • Create tests