Extend the environment (#156)

* Provide the code with more detailed documentation comments

* Fix typo

* Refactor the code a bit

* Use the right environment key

* Add a conditional statement to handle environment specific conditions

* Add a loop statement to handle environment specific sequences

* Make string interpolation of a environment value possible

* Make environmentvalue conditionable

* Refactor the code a bit

* Avoid type casting by using a type erasure

* Test the error reporting for the environment through the provider

* Make environment statement and sequence a global element

* Introduce a method to unwrap optional environment values

* Tidy up a bit

* Rename the method for the conditional statement

Sources/HTMLKit/Framework/Environment/Condition.swift

@@ -0,0 +1,42 @@
+/// A type representing a conditional that compares an environment value against another value
+@_documentation(visibility: internal)
+public struct Condition {
+
+    /// A enumeration of potential comparison
+    public enum Comparison {
+
+        /// Indicates an equal comparison
+        case equal
+
+        /// Indicates a not-equal comparison
+        case unequal
+
+        /// Indicates a greater-than comparison
+        case greater
+
+        /// Indicates a less-than comparison
+        case less
+    }
+
+    /// The left-hand side value
+    internal let lhs: EnvironmentValue
+
+    /// The right-hand side value to test against
+    internal let rhs: any Comparable
+
+    /// The comparison to perfom
+    internal let comparison: Comparison
+
+    /// Initializes a condition
+    ///
+    /// - Parameters:
+    ///   - lhs: The origin value
+    ///   - rhs: The value to atest against
+    ///   - operation: The comparison to perfom
+    public init(lhs: EnvironmentValue, rhs: any Comparable, comparison: Comparison) {
+
+        self.lhs = lhs
+        self.rhs = rhs
+        self.comparison = comparison
+    }
+}

Sources/HTMLKit/Framework/Environment/Conditional.swift

@@ -0,0 +1,19 @@
+/// A type that defines a conditonal value which will be evualuated by the renderer.
+@_documentation(visibility: internal)
+public indirect enum Conditional {
+
+    /// Holds an optional
+    case optional(EnvironmentValue)
+
+    /// Holds an relation
+    case relation(Relation)
+
+    /// Holds a condition
+    case condition(Condition)
+
+    /// Holds a negation
+    case negation(Negation)
+
+    /// Holds a value
+    case value(EnvironmentValue)
+}