fix range semantic in detect_columns, str substring, str index-of (#12894)

# Description
Fixes: https://github.com/nushell/nushell/issues/7761

It's still unsure if we want to change the `range semantic` itself, but
it's good to keep range semantic consistent between nushell commands.

# User-Facing Changes
### Before
```nushell
❯ "abc" | str substring 1..=2
b
```
### After
```nushell
❯ "abc" | str substring 1..=2
bc
```

# Tests + Formatting
Adjust tests to fit new behavior

crates/nu-command/src/strings/str_/index_of.rs

@@ -405,7 +405,7 @@ mod tests {
         let range = Range::new(
             Value::int(0, Span::test_data()),
             Value::int(1, Span::test_data()),
-            Value::int(3, Span::test_data()),
+            Value::int(2, Span::test_data()),
             RangeInclusion::Inclusive,
             Span::test_data(),
         )

crates/nu-command/src/strings/str_/substring.rs

@@ -70,7 +70,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Get part of a string. Note that the start is included but the end is excluded, and that the first character of a string is index 0."
+        "Get part of a string. Note that the first character of a string is index 0."
     }
 
     fn search_terms(&self) -> Vec<&str> {
@@ -108,12 +108,12 @@ impl Command for SubCommand {
             Example {
                 description:
                     "Get a substring \"nushell\" from the text \"good nushell\" using a range",
-                example: " 'good nushell' | str substring 5..12",
+                example: " 'good nushell' | str substring 5..11",
                 result: Some(Value::test_string("nushell")),
             },
             Example {
                 description: "Count indexes and split using grapheme clusters",
-                example: " '🇯🇵ほげ ふが ぴよ' | str substring --grapheme-clusters 4..6",
+                example: " '🇯🇵ほげ ふが ぴよ' | str substring --grapheme-clusters 4..5",
                 result: Some(Value::test_string("ふが")),
             },
         ]