Add isstoredband to check if a band is stored as a vector

[jishnub]

For example,

julia> B = Bidiagonal(1:3, 1:2, :U)
3×3 Bidiagonal{Int64, UnitRange{Int64}}:
 1  1  ⋅
 ⋅  2  2
 ⋅  ⋅  3

julia> LinearAlgebra.isstoredband(B, 0)
true

julia> LinearAlgebra.isstoredband(B, -1)
false

julia> L = LowerTriangular(B)
3×3 LowerTriangular{Int64, Bidiagonal{Int64, UnitRange{Int64}}}:
 1  ⋅  ⋅
 0  2  ⋅
 0  0  3

julia> LinearAlgebra.isstoredband(B, -1)
false

julia> LinearAlgebra.isstoredband(B, 0)
true

This is expected to be used in combination with #1424, such that diag(A, Val(k)) would return the underlying band and be non-allocating if isstoredband(A, k) returns true. As of now, this is an internal function that may be useful in #1423.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.88%. Comparing base (1f55352) to head (37090e0).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/dense.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1433      +/-   ##
==========================================
- Coverage   93.89%   93.88%   -0.01%     
==========================================
  Files          34       34              
  Lines       15900    15909       +9     
==========================================
+ Hits        14929    14936       +7     
- Misses        971      973       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.