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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helps us keep track of breaking changes. -->

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'"),
         }
     }
 }

crates/nu-table/src/common.rs

@@ -186,6 +186,7 @@ pub fn load_theme(mode: TableMode) -> TableTheme {
         TableMode::Restructured => TableTheme::restructured(),
         TableMode::AsciiRounded => TableTheme::ascii_rounded(),
         TableMode::BasicCompact => TableTheme::basic_compact(),
+        TableMode::Single => TableTheme::single(),
     }
 }
 

crates/nu-table/src/table_theme.rs

@@ -156,6 +156,16 @@ impl TableTheme {
         Self::new(theme, full)
     }
 
+    pub fn single() -> TableTheme {
+        let full = Style::modern()
+            .corner_top_left('┌')
+            .corner_top_right('┐')
+            .corner_bottom_left('└')
+            .corner_bottom_right('┘');
+
+        Self::new(Style::sharp(), full)
+    }
+
     pub fn none() -> TableTheme {
         Self::new(Style::blank(), Style::blank())
     }

crates/nu-table/tests/style.rs

@@ -451,6 +451,52 @@ fn test_with_love() {
     assert_eq!(create_table_with_size(vec![], true, theme::with_love()), "");
 }
 
+#[test]
+fn test_single() {
+    assert_eq!(
+        create_table(vec![row(4); 3], true, theme::single()),
+        "┌───┬───┬───┬───┐\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         ├───┼───┼───┼───┤\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         └───┴───┴───┴───┘"
+    );
+
+    assert_eq!(
+        create_table(vec![row(4); 2], true, theme::single()),
+        "┌───┬───┬───┬───┐\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         ├───┼───┼───┼───┤\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         └───┴───┴───┴───┘"
+    );
+
+    assert_eq!(
+        create_table(vec![row(4); 1], true, theme::single()),
+        "┌───┬───┬───┬───┐\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         └───┴───┴───┴───┘"
+    );
+
+    assert_eq!(
+        create_table(vec![row(4); 1], false, theme::single()),
+        "┌───┬───┬───┬───┐\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         └───┴───┴───┴───┘"
+    );
+
+    assert_eq!(
+        create_table(vec![row(4); 2], false, theme::single()),
+        "┌───┬───┬───┬───┐\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         │ 0 │ 1 │ 2 │ 3 │\n\
+         └───┴───┴───┴───┘"
+    );
+
+    assert_eq!(create_table_with_size(vec![], true, theme::single()), "");
+}
+
 fn create_table(data: Vec<Vec<Text<String>>>, with_header: bool, theme: theme) -> String {
     let mut case = TestCase::new(usize::MAX).theme(theme);
     if with_header {

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"