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/ast/unit.rs

@@ -32,7 +32,7 @@ pub enum Unit {
 }
 
 impl Unit {
-    pub fn to_value(&self, size: i64, span: Span) -> Result<Value, ShellError> {
+    pub fn build_value(self, size: i64, span: Span) -> Result<Value, ShellError> {
         match self {
             Unit::Byte => Ok(Value::filesize(size, span)),
             Unit::Kilobyte => Ok(Value::filesize(size * 1000, span)),