Restructure nu-protocol in more meaningful units (#11917)

This is partially "feng-shui programming" of moving things to new
separate places.

The later commits include "`git blame` tollbooths" by moving out chunks
of code into new files, which requires an extra step to track things
with `git blame`. We can negiotiate if you want to keep particular
things in their original place.

If egregious I tried to add a bit of documentation. If I see something
that is unused/unnecessarily `pub` I will try to remove that.


- Move `nu_protocol::Exportable` to `nu-parser`
- Guess doccomment for `Exportable`
- Move `Unit` enum from `value` to `AST`
- Move engine state `Variable` def into its folder
- Move error-related files in `nu-protocol` subdir
- Move `pipeline_data` module into its own folder
- Move `stream.rs` over into the `pipeline_data` mod
- Move `PipelineMetadata` into its own file
- Doccomment `PipelineMetadata`
- Remove unused `is_leap_year` in `value/mod`
- Note about criminal `type_compatible` helper
- Move duration fmting into new `value/duration.rs`
- Move filesize fmting logic to new `value/filesize`
- Split reexports from standard imports in `value/mod`
- Doccomment trait `CustomValue`
- Polish doccomments and intradoc links

crates/nu-protocol/src/errors/parse_warning.rs

@@ -0,0 +1,23 @@
+use crate::Span;
+use miette::Diagnostic;
+use serde::{Deserialize, Serialize};
+use thiserror::Error;
+
+#[derive(Clone, Debug, Error, Diagnostic, Serialize, Deserialize)]
+pub enum ParseWarning {
+    #[error("Deprecated: {0}")]
+    DeprecatedWarning(
+        String,
+        String,
+        #[label = "`{0}` is deprecated and will be removed in 0.90. Please use `{1}` instead, more info: https://www.nushell.sh/book/custom_commands.html"]
+        Span,
+    ),
+}
+
+impl ParseWarning {
+    pub fn span(&self) -> Span {
+        match self {
+            ParseWarning::DeprecatedWarning(_, _, s) => *s,
+        }
+    }
+}