Add dfr to dataframe cmds (#7998)

# Description

This PR tries to resolve the overloading issue by going back to our
original naming convention for dataframes. So, this PR renames all
dataframe commands with a prefix of `dfr`. Some commands like `open-df`
were renamed to `dfr open` and things like `into df` were renamed `dfr
into-df`. I'm sure we can optimize naming a bit, but it seems to compile
now.

# User-Facing Changes

All dataframe commands are prefixed with dfr.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

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

@@ -214,8 +214,8 @@ fn parses_arrow_ipc() {
     let actual = nu!(
         cwd: "tests/fixtures/formats", pipeline(
         r#"
-            open-df caco3_plastics.arrow
-            | into nu
+            dfr open caco3_plastics.arrow
+            | dfr into-nu
             | first
             | get origin
         "#