[Docs] Update library minimum target 1.3.2 -> 1.3.3

[Docs] Update minimum requirements in README.md

[Style] Change func naming from isFieldDuplicated to isDocumentExists

[Docs] Update README.md

[Docs] Update minimum requirements 1.2.8 -> 1.2.9

[Docs] Update minimum target version 1.2.7 -> 1.2.8

[Style] reorganize combine logic from SHFirestoreHelpers.swift to Com…

…bine+Helpers

[Docs] Update SHFirestoreMinimumTarget version in README.md

[Docs] Update spm target 1.2.5 in README.md

[Feat] Add FirestoreAccessible's extension helper computed properties…

… for casting as collection or document ref.