Rewrite which (#1144)

* Detect built-in commands passed as args to `which`

This expands the built-in `which` command to detect nushell commands
that may have the same name as a binary in the path.

* Allow which to interpret multiple arguments

Previously, it would discard any argument besides the first. This allows
`which` to process multiple arguments. It also makes the output a stream
of rows.

* Use map to build the output

* Add boolean column for builtins

* Use macros for entry creation shortcuts

* Process command args and use async_stream

In order to use `ichwh`, I'll need to use async_stream. But in order to
avoid lifetime errors with that, I have to process the command args
before using them. I'll admit I don't fully understand what is going on
with the `args.process(...)` function, but it works.

* Use `ichwh` for path searching

This commit transitions from `which` to `ichwh`. The path search is now
done asynchronously.

* Enable the `--all` flag on `which`

* Make `which` respect external commands

Escaped commands passed to wich (e.g., `which "^ls"`), are now searched
before builtins.

* Fix clippy warnings

This commit resolves two warnings from clippy, in light of #1142.

* Update Cargo.lock to get new `ichwh` version

`ichwh@0.2.1` has support for local paths.

* Add documentation for command

docs/commands/which.md

@@ -0,0 +1,72 @@
+# which
+
+Finds a program file.
+
+Usage:
+  > which  ...args{flags}
+
+## Parameters
+
+- ...args: the names of the commands to find the path to
+- --all: list all executables
+
+## Examples
+
+`which` finds the location of an executable:
+
+```shell
+/home/bob> which python
+━━━━━━━━┯━━━━━━━━━━━━━━━━━┯━━━━━━━━━
+ arg    │ path            │ builtin
+────────┼─────────────────┼─────────
+ python │ /usr/bin/python │ No
+━━━━━━━━┷━━━━━━━━━━━━━━━━━┷━━━━━━━━━
+/home/bob> which cargo
+━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━
+ arg   │ path                       │ builtin
+───────┼────────────────────────────┼─────────
+ cargo │ /home/bob/.cargo/bin/cargo │ No
+━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━
+```
+
+`which` will identify nushell commands:
+
+```shell
+/home/bob> which ls
+━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━
+ arg │ path                     │ builtin
+─────┼──────────────────────────┼─────────
+ ls  │ nushell built-in command │ Yes
+━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━
+/home/bob> which which
+━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━
+ arg   │ path                     │ builtin
+───────┼──────────────────────────┼─────────
+ which │ nushell built-in command │ Yes
+━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━
+```
+
+Passing the `all` flag identifies all instances of a command or binary
+
+```shell
+/home/bob> which ls --all
+━━━┯━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━
+ # │ arg │ path                     │ builtin
+───┼─────┼──────────────────────────┼─────────
+ 0 │ ls  │ nushell built-in command │ Yes
+ 1 │ ls  │ /usr/bin/ls              │ No
+━━━┷━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━
+```
+
+`which` will also identify local binaries
+
+```shell
+/home/bob> touch foo
+/home/bob> chmod +x foo
+/home/bob> which ./foo
+━━━━━━━┯━━━━━━━━━━━━━━━┯━━━━━━━━━
+ arg   │ path          │ builtin
+───────┼───────────────┼─────────
+ ./foo │ /home/bob/foo │ No
+━━━━━━━┷━━━━━━━━━━━━━━━┷━━━━━━━━━
+```