Create public append() function

[peterdesmet]

Fix #198

Originally written by @PietrH in #362. Edited text from original PR:

I ended up implementing a generic S3 method for append() which allowed me to set a S3 method for append.datapackage().

Converting append() to a generic S3 method will result in a message when loading frictionless:

Attaching package: ‘frictionless’

The following object is masked from ‘package:base’:

    append

This can be avoided by not using S3 dispatch at all, and using the internal helper for packages as well. But then users will not be able to call append() on a datapackage without dropping attributes.

TODO

• Add tests
• Do we need to use base::append() in append_with_attributes() tests?
• Merge append_with_attrs() functionality into this branch
• Search for append in documentation and update (e.g. in vignettes)
• Assign a function family and decide where to list in pkgdown

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.87%. Comparing base (c49bc26) to head (ad0813e).

Files with missing lines	Patch %	Lines
R/append.datapackage.R	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #365      +/-   ##
===========================================
- Coverage   100.00%   99.87%   -0.13%     
===========================================
  Files           28       29       +1     
  Lines          827      830       +3     
===========================================
+ Hits           827      829       +2     
- Misses           0        1       +1     

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