Add support for definite integration

[GregVernon]

See this Discourse conversation: https://discourse.julialang.org/t/integrating-symbolic-functions/102545

Thought I'd start a basic MR with the most naive implementation, and let it evolve from here. I've only done the most basic manual testing (e.g. integrate $x$ over the domain [0,1]), but am open to suggestions for how to build out testing -- is it enough to rely on Symbolics.substitute unit tests, and then just a few return-type / edge case tests?

[ChrisRackauckas]

is it enough to rely on Symbolics.substitute unit tests, and then just a few return-type / edge case tests?

It would be good to test a few values and cases.

[codecov[bot]]

Codecov Report

Merging #59 (ca1a14b) into main (ae77a3d) will decrease coverage by 64.87%. The diff coverage is 0.00%.

:exclamation: Current head ca1a14b differs from pull request most recent head 9aaa42e. Consider uploading reports for the commit 9aaa42e to get more accurate results

@@            Coverage Diff             @@
##             main     #59       +/-   ##
==========================================
- Coverage   64.96%   0.10%   -64.87%     
==========================================
  Files          14      14               
  Lines         942     933        -9     
==========================================
- Hits          612       1      -611     
- Misses        330     932      +602     

Files Changed	Coverage Δ
src/integral.jl	0.00% <0.00%> (-80.51%)	:arrow_down:

... and 10 files with indirect coverage changes

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[knuesel]

I think this can be closed as superseded by https://github.com/SciML/SymbolicNumericIntegration.jl/pull/63 ?