Add BeASlice and BeAnArray matchers

[sanbricio]

Summary

Implements BeASlice() and BeAnArray() matchers as requested in #584.

These matchers enable type assertions for slice and array types when working with interface{} values:

• BeASlice(): Succeeds if actual is a slice type ([]T)
• BeAnArray(): Succeeds if actual is an array type ([N]T)

Examples

var data any = []int{1, 2, 3}
Expect(data).Should(BeASlice())

var arr any = [3]int{1, 2, 3}
Expect(arr).Should(BeAnArray())

Expect("hello").ShouldNot(BeASlice())  // string is not a slice

Implementation

• Added BeASliceMatcher and BeAnArrayMatcher structs in matchers package
• Added constructor functions BeASlice() and BeAnArray() in root matchers.go
• Full test coverage including:
  • Multiple slice/array element types
  • Nil slice handling
  • Type validation (error on nil literal)
  • Failure message verification
  • Negation tests

Testing

All 524 matchers tests pass
go vet ./... passes with no warnings
9 new specs covering both matchers