Add multiline regex flag to str replace (#9508)

# Description

This change adds a new flag to the `str replace` command: `--multiline`,
`-m`. This flag will automatically add the multiline regex flag `(?m)`
to the beginning of the supplied regex, allowing for the `^` and `$`
regex characters to match the beginnings and ends of lines,
respectively.

The main advantage of this addition is to make `str replace` more
closely match sed's default behavior of performing matches per-line with
a simple cli flag as opposed to forcing the user to add the {somewhat
clunky) regex flag themselves. This could be an especially valuable
addition since [`str replace` is listed as an alternative to sed in the
official
documentation](https://www.nushell.sh/book/coming_from_bash.html).

With this change, the following two commands would be functionally
equivalent:

```bash
# bash
printf "non-matching line\n123. one line\n124. another line\n" | sed -r 's/^[0-9]+\. //'
```

```bash
# nu
"non-matching line\n123. one line\n124. another line\n" | str replace -am '^[0-9]+\. ' ''
```

both producing the following output:

```
non-matching line
one line
another line
```

# User-Facing Changes

1. Adds a new flag to the `str replace` command: `--multiline`, `-m`.

# Tests + Formatting

I have update the unit tests to test this flag.
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crates/nu-command/src/strings/str_/replace.rs

@@ -15,6 +15,7 @@ struct Arguments {
     cell_paths: Option<Vec<CellPath>>,
     literal_replace: bool,
     no_regex: bool,
+    multiline: bool,
 }
 
 impl CmdArgument for Arguments {
@@ -53,6 +54,11 @@ impl Command for SubCommand {
                 "match the pattern as a substring of the input, instead of a regular expression",
                 Some('s'),
             )
+            .switch(
+                "multiline",
+                "multi-line regex mode: ^ and $ match begin/end of line; equivalent to (?m)",
+                Some('m'),
+            )
             .category(Category::Strings)
     }
 
@@ -77,6 +83,7 @@ impl Command for SubCommand {
         let cell_paths = (!cell_paths.is_empty()).then_some(cell_paths);
         let literal_replace = call.has_flag("no-expand");
         let no_regex = call.has_flag("string");
+        let multiline = call.has_flag("multiline");
 
         let args = Arguments {
             all: call.has_flag("all"),
@@ -85,6 +92,7 @@ impl Command for SubCommand {
             cell_paths,
             literal_replace,
             no_regex,
+            multiline,
         };
         operate(action, args, input, call.head, engine_state.ctrlc.clone())
     }
@@ -148,6 +156,11 @@ impl Command for SubCommand {
                 example: r#"'GHIKK-9+*' | str replace '[*[:xdigit:]+]' 'z'"#,
                 result: Some(Value::test_string("GHIKK-z+*")),
             },
+            Example {
+                description: "Find and replace on individual lines (multiline)",
+                example: r#""non-matching line\n123. one line\n124. another line\n" | str replace -am '^[0-9]+\. ' ''"#,
+                result: Some(Value::test_string("non-matching line\none line\nanother line\n")),
+            },
 
         ]
     }
@@ -163,6 +176,7 @@ fn action(
         all,
         literal_replace,
         no_regex,
+        multiline,
         ..
     }: &Arguments,
     head: Span,
@@ -185,7 +199,12 @@ fn action(
                 }
             } else {
                 // use regular expressions to replace strings
-                let regex = Regex::new(find_str);
+                let flags = match multiline {
+                    true => "(?m)",
+                    false => "",
+                };
+                let regex_string = flags.to_string() + find_str;
+                let regex = Regex::new(&regex_string);
 
                 match regex {
                     Ok(re) => {
@@ -264,6 +283,7 @@ mod tests {
             literal_replace: false,
             all: false,
             no_regex: false,
+            multiline: false,
         };
 
         let actual = action(&word, &options, Span::test_data());