Generate shell completions for `nextonic`

[bryango]

This is made possible by recent cleanups #1162, #1167, #1169 of the cli implementations. Thank you all! @rm-dr @CraftSpider

The completions are generated by clap_complete. The current v1 & v2 cli mixture makes it a bit difficult to offer clean completions for the tectonic command, so for now we only target the nextonic command exclusively for the v2 cli. To print the shell completions, simply run:

tectonic -X show shell-completions zsh # or bash, or your favorite <SHELL>

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 46.95%. Comparing base (82484db) to head (c0e57c0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1171      +/-   ##
==========================================
+ Coverage   46.92%   46.95%   +0.02%     
==========================================
  Files         177      177              
  Lines       65160    65179      +19     
==========================================
+ Hits        30575    30602      +27     
+ Misses      34585    34577       -8     

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[bryango]

Documented shell completions in the book. Aside: there are other useful information that tectonic -X show can provide, e.g. the hardcoded bundle url. I can work on this in a future PR.