add Enzyme to "extras" for testing, docs

[stevengj]

This adds the new weakdep Enzyme to the "extras" section and for testing.

Also adds a compat entry for the stdlib LinearAlgebra since that is required nowadays.

bumps the Julia version requirement to 1.6 since that is needed by Enzyme (a test dependency even though it is optional for installation), and it is a pain to continue to support 1.2 with this dependency present. Not much point in supporting ancient Julia versions these days.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.23%. Comparing base (b8ee1d2) to head (77c211b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
- Coverage   93.43%   92.23%   -1.21%     
==========================================
  Files           8        8              
  Lines         762      773      +11     
==========================================
+ Hits          712      713       +1     
- Misses         50       60      +10     

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[stevengj]

Looks like Julia's inference is failing for to_segbuf on Julia 1.6–1.8. I'm not sure it's worth it to fix this — it still works but is suboptimal on old Julia versions (though the test passed in Julia 1.2 IIRC!), and is type-stable on newer Julia versions. Better to just disable the test for versions < 1.9.