Turn Groebner.jl into an extension

[sumiya11]

Implements dispatch for calling groebner_basis/is_groebner_basis on Vector{Num}.

Also fixes some deprecation warnings.
Note that Groebner.jl v0.6.0+ is licensed under GNU GPL v2.

using Symbolics
@variables x1 x2 x3

groebner_basis([x1 + x2 + x3, x1*x2 + x1*x3 + x2*x3, x1*x2*x3 - 1])
# Throws
ERROR: "Groebner bases engine is required. Execute `using Groebner` to enable this functionality."

using Groebner
groebner_basis([x1 + x2 + x3, x1*x2 + x1*x3 + x2*x3, x1*x2*x3 - 1])
# Returns
3-element Vector{PolyForm{Real}}:
 x3 + x2 + x1
 (x3^2) + x2*x3 + (x2^2)
 -1 + (x3^3)

We can maybe wrap the resulting polynomials so that it returns Num..

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5806b2b) 77.61% compared to head (add3319) 9.77%.

Files	Patch %	Lines
src/Symbolics.jl	0.00%	2 Missing ⚠️
src/groebner_basis.jl	60.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master   #1035       +/-   ##
==========================================
- Coverage   77.61%   9.77%   -67.85%     
==========================================
  Files          29      30        +1     
  Lines        3422    3366       -56     
==========================================
- Hits         2656     329     -2327     
- Misses        766    3037     +2271     

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