Fix scitype for multisets of tuples of strings (and tagged strings)

[ablaom]

This used to return Unknown:

julia> bag_of_ngrams =
Dict(
  ("cat",)       => 1,
  ("cat", "in")  => 1,
  ("in",)        => 1,
  ("the", "hat") => 1,
  ("the",)       => 2,
  ("hat",)       => 1,
  ("in", "the")  => 1,
  ("the", "cat") => 1)

julia> scitype(bag_of_ngrams)
Multiset{Tuple{Vararg{Textual, var"#s13"}} where var"#s13"}

[codecov-commenter]

Codecov Report

Merging #158 (d2ff22e) into dev (8bb610c) will decrease coverage by 1.50%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              dev     #158      +/-   ##
==========================================
- Coverage   92.02%   90.52%   -1.51%     
==========================================
  Files           7        7              
  Lines         301      306       +5     
==========================================
  Hits          277      277              
- Misses         24       29       +5     

Impacted Files	Coverage Δ
src/convention/scitype.jl	58.00% <0.00%> (-6.45%)	:arrow_down:

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