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-engine/src/env.rs

@@ -353,7 +353,7 @@ pub fn find_in_dirs_env(
 /// is the canonical way to fetch config at runtime when you have Stack available.
 pub fn get_config(engine_state: &EngineState, stack: &Stack) -> Config {
     if let Some(mut config_record) = stack.get_env_var(engine_state, "config") {
-        config_record.into_config(engine_state.get_config()).0
+        config_record.parse_as_config(engine_state.get_config()).0
     } else {
         engine_state.get_config().clone()
     }

crates/nu-engine/src/eval.rs

@@ -314,7 +314,7 @@ pub fn eval_expression_with_input<D: DebugContext>(
 
 // Try to catch and detect if external command runs to failed.
 fn might_consume_external_result(input: PipelineData) -> (PipelineData, bool) {
-    input.is_external_failed()
+    input.check_external_failed()
 }
 
 fn eval_redirection<D: DebugContext>(