Add command_prelude module (#12291)

# Description
When implementing a `Command`, one must also import all the types
present in the function signatures for `Command`. This makes it so that
we often import the same set of types in each command implementation
file. E.g., something like this:
```rust
use nu_protocol::ast::Call;
use nu_protocol::engine::{Command, EngineState, Stack};
use nu_protocol::{
    record, Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData,
    ShellError, Signature, Span, Type, Value,
};
```

This PR adds the `nu_engine::command_prelude` module which contains the
necessary and commonly used types to implement a `Command`:
```rust
// command_prelude.rs
pub use crate::CallExt;
pub use nu_protocol::{
    ast::{Call, CellPath},
    engine::{Command, EngineState, Stack},
    record, Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, IntoSpanned,
    PipelineData, Record, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
};
```

This should reduce the boilerplate needed to implement a command and
also gives us a place to track the breadth of the `Command` API. I tried
to be conservative with what went into the prelude modules, since it
might be hard/annoying to remove items from the prelude in the future.
Let me know if something should be included or excluded.

crates/nu-parser/src/parser.rs

@@ -1,41 +1,27 @@
 use crate::{
     lex::{lex, lex_signature},
     lite_parser::{lite_parse, LiteCommand, LitePipeline, LiteRedirection, LiteRedirectionTarget},
+    parse_keywords::*,
     parse_mut,
     parse_patterns::parse_pattern,
     parse_shape_specs::{parse_shape_name, parse_type, ShapeDescriptorUse},
     type_check::{self, math_result_type, type_compatible},
     Token, TokenContents,
 };
-
-use nu_engine::DIR_VAR_PARSER_INFO;
-use nu_protocol::{
-    ast::{
-        Argument, Assignment, Bits, Block, Boolean, Call, CellPath, Comparison, Expr, Expression,
-        ExternalArgument, FullCellPath, ImportPattern, ImportPatternHead, ImportPatternMember,
-        MatchPattern, Math, Operator, PathMember, Pattern, Pipeline, PipelineElement,
-        PipelineRedirection, RangeInclusion, RangeOperator, RecordItem, RedirectionTarget,
-    },
-    engine::StateWorkingSet,
-    eval_const::eval_constant,
-    span, BlockId, DidYouMean, Flag, ParseError, PositionalArg, Signature, Span, Spanned,
-    SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
-};
-
-use crate::parse_keywords::{
-    find_dirs_var, is_unaliasable_parser_keyword, parse_alias, parse_const, parse_def,
-    parse_def_predecl, parse_export_in_block, parse_extern, parse_for, parse_hide, parse_keyword,
-    parse_let, parse_module, parse_overlay_hide, parse_overlay_new, parse_overlay_use,
-    parse_source, parse_use, parse_where, parse_where_expr, LIB_DIRS_VAR,
-};
-
 use itertools::Itertools;
 use log::trace;
+use nu_engine::DIR_VAR_PARSER_INFO;
+use nu_protocol::{
+    ast::*, engine::StateWorkingSet, eval_const::eval_constant, span, BlockId, DidYouMean, Flag,
+    ParseError, PositionalArg, Signature, Span, Spanned, SyntaxShape, Type, Unit, VarId,
+    ENV_VARIABLE_ID, IN_VARIABLE_ID,
+};
 use std::{
     collections::{HashMap, HashSet},
+    num::ParseIntError,
+    str,
     sync::Arc,
 };
-use std::{num::ParseIntError, str};
 
 #[cfg(feature = "plugin")]
 use crate::parse_keywords::parse_register;