Add url decode command (#10611)

Implemented URL decoding as a url subcommand, created corresponding unit
tests. The logic, examples and descriptions were based on the existing
`url encode` command.

Resolves #10563

# Description
Added a new `url decode` command to compliment the existing `url
encode`, as proposed by myself in #10563.
It takes a string, list of strings or cell path and produces the
corresponding decoded strings.

![image](https://github.com/nushell/nushell/assets/4030336/815a34e9-7ceb-4d09-9d74-e700ba513b17)

# User-Facing Changes
New url subcommand `url decode`, as described above.

# Tests + Formatting
I've added unit tests for the new subcommand and ensured all actions
outlined below showed no issues.
- [x] `cargo fmt --all -- --check`
- [x] `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used`
- [x] `cargo test --workspace`
- [x] `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"`

crates/nu-command/src/default_context.rs

@@ -357,6 +357,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             HttpOptions,
             Url,
             UrlBuildQuery,
+            UrlDecode,
             UrlEncode,
             UrlJoin,
             UrlParse,