Refactor find to handle regex search and non-regex search the same way (#15839)

# Description

Regex search and search with directly provided search terms used to
follow two different code paths. Now all possible search options get
turned into a regex, with optional additional search options, and
handled using a unified code path which mostly follows the logic of the
current term code path.

# User-Facing Changes

Regex search will now behave in the same way as non-regex search:
- split multiline strings into lists of lines, and filter out the lines
that don't match
- highlight matching string sections (unless --no-highlight flag is
used)
- search through the specified record columns if the --columns flag is
used

The behavior of non-regex search should be unaffected by this commit.

crates/nu-command/tests/commands/find.rs

@@ -118,7 +118,10 @@ fn find_with_regex_in_table_keeps_row_if_one_column_matches() {
         "[[name nickname]; [Maurice moe] [Laurence larry]] | find --no-highlight --regex ce | get name | to json -r"
     );
 
-    assert_eq!(actual.out, r#"["Maurice","Laurence"]"#);
+    assert_eq!(
+        actual.out,
+        r#"["\u001b[37mMauri\u001b[0m\u001b[41;37mce\u001b[0m\u001b[37m\u001b[0m","\u001b[37mLauren\u001b[0m\u001b[41;37mce\u001b[0m\u001b[37m\u001b[0m"]"#
+    );
     assert_eq!(actual_no_highlight.out, r#"["Maurice","Laurence"]"#);
 }