feat(table): Add new 'single' table mode (#15672)

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closes #15381

# Description
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Adds a new table mode called `single`, it looks like the `heavy` mode,
but the key difference is that it uses thinner lines. I decided on the
name `single` because it's one of the border styles Neovim uses, and
they look practically the same.

# User-Facing Changes
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New config option:

```nushell
$env.config.table.mode = 'single'
```

# Tests + Formatting
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Added new tests in `crates/nu-table/tests/style.rs` to cover the single
table mode.

# After Submitting
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crates/nu-utils/src/default_files/doc_config.nu

@@ -300,7 +300,7 @@ $env.config.footer_mode = 25
 # Specifies the visual display style of a table
 # One of: "default", "basic", "compact", "compact_double", "heavy", "light", "none", "reinforced",
 # "rounded", "thin", "with_love", "psql", "markdown", "dots", "restructured", "ascii_rounded",
-# or "basic_compact"
+# "basic_compact" or "single"
 # Can be overridden by passing a table to `| table --theme/-t`
 $env.config.table.mode = "default"