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/engine/pattern_match.rs

@@ -145,7 +145,7 @@ impl Matcher for Pattern {
                         let span = unit.span;
 
                         if let Expr::Int(size) = amount.expr {
-                            match &unit.item.to_value(size, span) {
+                            match &unit.item.build_value(size, span) {
                                 Ok(v) => v == value,
                                 _ => false,
                             }