Follow API guidelines for public types (#12283)

# Description
Follow the [API guideline naming
conventions](https://rust-lang.github.io/api-guidelines/naming.html)
also for our externally exposed types

(See
[`clippy::wrong_self_convention`](https://rust-lang.github.io/rust-clippy/master/index.html#/wrong_self_convention)
with [`avoid-breaking-exported-api =
false`](https://doc.rust-lang.org/clippy/lint_configuration.html#avoid-breaking-exported-api)
)

Also be a good citizen around doccomments

- **Fix `Unit::to_value` to `Unit::build_value`**
- **Fix `PipelineData::is_external_failed` to `check_external_failed`**
- **Fix doccomment on `check_external_failed`**
- **Fix `Value::into_config` naming to `parse_as_config`**
- **Document `Value::parse_as_config`**

# Plugin-Author-Facing Changes
See renames above

crates/nu-protocol/src/eval_base.rs

@@ -136,7 +136,7 @@ pub trait Eval {
             Expr::String(s) => Ok(Value::string(s.clone(), expr.span)),
             Expr::Nothing => Ok(Value::nothing(expr.span)),
             Expr::ValueWithUnit(e, unit) => match Self::eval::<D>(state, mut_state, e)? {
-                Value::Int { val, .. } => unit.item.to_value(val, unit.span),
+                Value::Int { val, .. } => unit.item.build_value(val, unit.span),
                 x => Err(ShellError::CantConvert {
                     to_type: "unit value".into(),
                     from_type: x.get_type().to_string(),