update char signature with Table (#9895)

# Description

This PR updates the `char` command to allow `Table` output due to the
`--list` parameter.

### Before
```nushell
char --list | transpose
Error: nu::parser::input_type_mismatch

  × Command does not support string input.
   ╭─[entry #6:1:1]
 1 │ char --list | transpose
   ·               ────┬────
   ·                   ╰── command doesn't support string input
   ╰────
```

### After
```nushell
❯ char --list | transpose
╭───┬───────────┬─────────┬─────────┬─────────┬───────────┬─────────┬─────────────┬─────────┬─────────┬─────────┬──────────┬──────────┬──────────┬────────────┬──────────┬─────────────┬──────────┬────────────┬──────────┬──────────┬─────╮
│ # │  column0  │ column1 │ column2 │ column3 │  column4  │ column5 │   column6   │ column7 │ column8 │ column9 │ column10 │ column11 │ column12 │  column13  │ column14 │  column15   │ column16 │  column17  │ column18 │ column19 │ ... │
├───┼───────────┼─────────┼─────────┼─────────┼───────────┼─────────┼─────────────┼─────────┼─────────┼─────────┼──────────┼──────────┼──────────┼────────────┼──────────┼─────────────┼──────────┼────────────┼──────────┼──────────┼─────┤
│ 0 │ name      │ newline │ enter   │ nl      │ line_feed │ lf      │ carriage_re │ cr      │ crlf    │ tab     │ sp       │ space    │ pipe     │ left_brace │ lbrace   │ right_brace │ rbrace   │ left_paren │ lp       │ lparen   │ ... │
│   │           │         │         │         │           │         │ turn        │         │         │         │          │          │          │            │          │             │          │            │          │          │     │
│ 1 │ character │         │         │         │           │         │             │         │         │         │          │          │ |        │ {          │ {        │ }           │ }        │ (          │ (        │ (        │ ... │
│   │           │         │         │         │           │         │             │         │         │         │          │          │          │            │          │             │          │            │          │          │     │
│ 2 │ unicode   │ a       │ a       │ a       │ a         │ a       │ d           │ d       │ d a     │ 9       │ 20       │ 20       │ 7c       │ 7b         │ 7b       │ 7d          │ 7d       │ 28         │ 28       │ 28       │ ... │
╰───┴───────────┴─────────┴─────────┴─────────┴───────────┴─────────┴─────────────┴─────────┴─────────┴─────────┴──────────┴──────────┴──────────┴────────────┴──────────┴─────────────┴──────────┴────────────┴──────────┴──────────┴─────╯
```

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->

# Tests + Formatting
<!--
Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect -A clippy::result_large_err` to check that
you're using the standard code style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"` to run the tests for the standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

# After Submitting
<!-- If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.
-->

crates/nu-command/src/strings/char_.rs

@@ -157,7 +157,10 @@ impl Command for Char {
 
     fn signature(&self) -> Signature {
         Signature::build("char")
-            .input_output_types(vec![(Type::Nothing, Type::String)])
+            .input_output_types(vec![
+                (Type::Nothing, Type::String),
+                (Type::Nothing, Type::Table(vec![])),
+            ])
             .optional(
                 "character",
                 SyntaxShape::Any,
@@ -167,6 +170,7 @@ impl Command for Char {
             .switch("list", "List all supported character names", Some('l'))
             .switch("unicode", "Unicode string i.e. 1f378", Some('u'))
             .switch("integer", "Create a codepoint from an integer", Some('i'))
+            .allow_variants_without_examples(true)
             .category(Category::Strings)
     }