2.0: add ExpectedTable variant so table accessors return WrongType

[tylerbutler]

Summary

Adding an ExpectedTable variant to ExpectedType so table-shaped accessors (e.g. the table_keys helper proposed in #23) can return a precise WrongType error when a path resolves to a non-table value.

Why this is breaking (and therefore 2.0)

ExpectedType documents its variants as part of the stable public API:

Variants are part of the stable public API. Adding, removing, or renaming a variant is treated as a breaking change.

So introducing ExpectedTable is a major-version change. Until then, accessors like table_keys (#23) must work around the gap — either reusing an existing ExpectedType value (inaccurate), collapsing the non-table case into KeyNotFound (lossy), or returning a bare List(String) and sidestepping GetError entirely.

Proposed change

pub type ExpectedType {
  ExpectedString
  ExpectedInt
  ExpectedBool
  ExpectedFloat
  ExpectedDate
  ExpectedTime
  ExpectedDateTime
  ExpectedTable   // new
}

This lets table_keys (and any future table-shaped accessor) return:

• missing key -> Error(KeyNotFound(path))
• present but not a table -> Error(WrongType(path, ExpectedTable))

matching the existing get_* family's error style.

Related

• Add table key enumeration helper (e.g. table_keys/doc, path) #23 (table key enumeration helper) — can ship non-breaking now; this issue tightens its error reporting in 2.0.