Add ShellWarning (#16147)

# Description
Adds a proper `ShellWarning` enum which has the same functionality as
`ParseWarning`.

Also moves the deprecation from #15806 into `ShellWarning::Deprecated`
with `ReportMode::FirstUse`, so that warning will only pop up once now.

# User-Facing Changes
Technically the change to the deprecation warning from #15806 is user
facing but it's really not worth listing in the changelog

crates/nu-cmd-base/src/hook.rs

@@ -3,9 +3,9 @@ use nu_engine::{eval_block, eval_block_with_early_return, redirect_env};
 use nu_parser::parse;
 use nu_protocol::{
     PipelineData, PositionalArg, ShellError, Span, Type, Value, VarId,
-    cli_error::{report_parse_error, report_shell_error},
     debugger::WithoutDebug,
     engine::{Closure, EngineState, Stack, StateWorkingSet},
+    report_error::{report_parse_error, report_shell_error},
 };
 use std::{collections::HashMap, sync::Arc};