Log Message:
shells/elvish: update to 0.19.0

Notable new features
-   A new `doc` module provides access to the documentation of builtin modules.
-   A new `conj` command "appends" values to a list, and has a \ 
guaranteed time
    complexity independent of the size of the list.
-   A new `inexact-num` converts its argument to an inexact number.
    It is functionally identical to the now deprecated `float64` command since
    the Go float64 type is the only underlying inexact number type for now. Its
    behavior may change in future if there are more underlying types for inexact
    numbers.
-   A new type of interactive abbreviation: `edit:command-abbr`
    ([#1472](https://b.elv.sh/1472)).
-   The `order` and `compare` commands now support boolean values
    ([#1585](https://b.elv.sh/1585)).
-   A new `path:join` command and `path:separator` and `path:list-separator`
    variables ([#1562](https://b.elv.sh/1562)).
-   A new `runtime:` module that contains paths important for the Elvish runtime
    ([#1385](https://b.elv.sh/1385), [#1423](https://b.elv.sh/1423)).
-   A new `compact` command that replaces consecutive runs of equal values with
    a single copy, similar to the Unix `uniq` command.
-   The `order` command has a new `&key` option
    ([#1570](https://b.elv.sh/1570)).
-   A new `benchmark` command has been added ([#1586](https://b.elv.sh/1586)).
-   When checking compilation errors, Elvish no longer stops after the first
    error found. For example, if `$a` and `$b` are both not defined,
    `echo $a $b` now yields two errors. This applies to both the interactive
    REPL and `elvish -compile-only`.
-   When using an unimported builtin modules from the REPL, the REPL now shows
    the `use` command needed to import it, which can be executed from a key
    binding. This functionality is bound to Ctrl-A by default.
-   New variables exposing the terminal and null device in an OS-agnostic
    fashion: `$path:dev-tty` and `$path:dev-null`. They are `/dev/tty` and
    `/dev/null` on Unix, and `CON` and `NUL` on Windows
    ([#1633](https://b.elv.sh/1633)).

Breaking changes
-   When a `styled` or `styled-segment` is printed to terminal, the resulting
    sequence will now always ignore any existing SGR state.
-   Symbolic links are now always treated as ordinary files by the global
    modifiers `type:dir` and `type:regular` in wildcard expansions.
-   Support for shared vars has been removed, along with its API
    (`store:shared-var`, `store:set-shared-var` and `store:del-shared-var`).

Deprecated features

Deprecated features will be removed in 0.20.0.

The following deprecated features trigger a warning whenever the code is parsed
and compiled, even if it is not executed:
-   The `float64` command is now deprecated. Use `num` for constructing a typed
    number, or `inexact-num` for constructing an inexact number.
    The documentation has advertised it as deprecated since the 0.16.0 release,
    but deprecation warnings were never added.

Notable bugfixes
-   Temporary assignment on an unset environment variables no longer leave it
    set to an empty string ([#1448](https://b.elv.sh/1448)).
-   Broken symbolic links no longer terminate a wildcard expansion prematurely
    ([#1240](https://b.elv.sh/1240)).
-   On Windows, command completion for executables now also works for local
    files