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-protocol/src/config/table.rs

@@ -20,6 +20,7 @@ pub enum TableMode {
     Restructured,
     AsciiRounded,
     BasicCompact,
+    Single,
 }
 
 impl FromStr for TableMode {
@@ -44,7 +45,8 @@ impl FromStr for TableMode {
             "restructured" => Ok(Self::Restructured),
             "ascii_rounded" => Ok(Self::AsciiRounded),
             "basic_compact" => Ok(Self::BasicCompact),
-            _ => Err("'basic', 'thin', 'light', 'compact', 'with_love', 'compact_double', 'rounded', 'reinforced', 'heavy', 'none', 'psql', 'markdown', 'dots', 'restructured', 'ascii_rounded', or 'basic_compact'"),
+            "single" => Ok(Self::Single),
+            _ => Err("'basic', 'thin', 'light', 'compact', 'with_love', 'compact_double', 'rounded', 'reinforced', 'heavy', 'none', 'psql', 'markdown', 'dots', 'restructured', 'ascii_rounded', 'basic_compact' or 'single'"),
         }
     }
 }