Implementing the --shadow-builtin flag has proven to be highly controversial.
Revert the introduction of that flag to the `function` command. If someone
shoots themselves in the foot by redefining a builtin as a function that's
their problem and not our responsibility to protect them from doing so.
Fixes#3319
It's currently too easy for someone to bork their shell by doing something
like `function test; return 0; end`. That's obviously a silly, contrived,
example but the point is that novice users who learn about functions are
prone to do something like that without realizing it will bork the shell. Even
expert users who know about the `test` builtin might forget that, say, `pwd`
is a builtin.
This change adds a `--shadow-builtin` flag that must be specified to
indicate you know what you're doing.
Fixes#3000
--inherit-variable takes a variable name and snapshots its current
value. When the function is executed, it will have a local variable with
this value already defined. Printing the function source will include
synthesized `set -l` lines for the values.
This is primarily useful for functions that are created on the fly, such
as in `psub`.
