Provide better errors on unknown type in cyclic definitions.

Let's consider the following case:

  ```
  type Var =
    Integer

  type Vars =
    List<Var>
  ```

  This incorrectly reports an infinite cycle; due to the inability to
  properly type-check `Var` which is also a dependent var of `Vars`. Yet
  the real issue here being that `Integer` is an unknown type.

  This commit also upgrades miette to 7.2.0, so that we can also display
  a better error output when the problem is actually a cycle.

crates/aiken-project/src/blueprint/error.rs

@@ -19,7 +19,7 @@ pub enum Error {
         #[label("invalid validator's boundary")]
         location: Span,
         #[source_code]
-        source_code: NamedSource,
+        source_code: NamedSource<String>,
     },
 
     #[error("Invalid or missing project's blueprint file.")]

crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__free_vars.snap

@@ -20,5 +20,6 @@ Schema {
     source_code: NamedSource {
         name: "",
         source: "<redacted>",
+        language: None,
     ,
 }

crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_multi_variants.snap

@@ -20,5 +20,6 @@ Schema {
     source_code: NamedSource {
         name: "",
         source: "<redacted>",
+        language: None,
     ,
 }

crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_variants.snap

@@ -51,5 +51,6 @@ Schema {
     source_code: NamedSource {
         name: "",
         source: "<redacted>",
+        language: None,
     ,
 }