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-cli/src/eval_cmds.rs

@@ -3,9 +3,9 @@ use nu_engine::eval_block;
 use nu_parser::parse;
 use nu_protocol::{
     PipelineData, ShellError, Spanned, Value,
-    cli_error::report_compile_error,
     debugger::WithoutDebug,
     engine::{EngineState, Stack, StateWorkingSet},
+    report_error::report_compile_error,
     report_parse_error, report_parse_warning,
 };
 use std::sync::Arc;

crates/nu-cli/src/eval_file.rs

@@ -5,9 +5,9 @@ use nu_parser::parse;
 use nu_path::canonicalize_with;
 use nu_protocol::{
     PipelineData, ShellError, Span, Value,
-    cli_error::report_compile_error,
     debugger::WithoutDebug,
     engine::{EngineState, Stack, StateWorkingSet},
+    report_error::report_compile_error,
     report_parse_error, report_parse_warning,
     shell_error::io::*,
 };

crates/nu-cli/src/util.rs

@@ -5,9 +5,9 @@ use nu_engine::{eval_block, eval_block_with_early_return};
 use nu_parser::{Token, TokenContents, lex, parse, unescape_unquote_string};
 use nu_protocol::{
     PipelineData, ShellError, Span, Value,
-    cli_error::report_compile_error,
     debugger::WithoutDebug,
     engine::{EngineState, Stack, StateWorkingSet},
+    report_error::report_compile_error,
     report_parse_error, report_parse_warning, report_shell_error,
 };
 #[cfg(windows)]