Allow the ordering to contain more variables than the input polynomials

[sumiya11]

With this PR, the following does not error:

using DynamicPolynomials, Groebner
@polyvar x y z
groebner([x, y], ordering=Lex(x, y, z))

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.27%. Comparing base (ff30fd1) to head (58ec3bc).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/monomials/orderings.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
- Coverage   94.29%   94.27%   -0.02%     
==========================================
  Files          41       41              
  Lines        5692     5695       +3     
==========================================
+ Hits         5367     5369       +2     
- Misses        325      326       +1     

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