feat(cli): add interactive repl

src/main/kotlin/lumen/cli/Repl.kt

@@ -0,0 +1,49 @@
+package lumen.cli
+
+import lumen.LumenError
+import lumen.compiler.Compiler
+import lumen.formatError
+import lumen.lexer.Lexer
+import lumen.module.ModuleError
+import lumen.module.ModuleLoader
+import lumen.parser.Parser
+import lumen.runtime.NilValue
+import lumen.runtime.displayString
+import lumen.vm.Vm
+import java.io.File
+
+class Repl {
+    private val vm = Vm()
+    private val globalMutability = mutableMapOf<String, Boolean>()
+    private val loader = ModuleLoader(vm, globalMutability)
+    private val origin = File("<repl>")
+
+    fun start() {
+        println("lumen repl - type 'exit' to quit")
+        val reader = System.`in`.bufferedReader()
+        while (true) {
+            print("lumen> ")
+            System.out.flush()
+            val line = reader.readLine() ?: break
+            val trimmed = line.trim()
+            if (trimmed.isEmpty()) continue
+            if (trimmed == "exit" || trimmed == "quit") break
+            evaluate(line)
+        }
+        println()
+    }
+
+    private fun evaluate(source: String) {
+        try {
+            val program = Parser(Lexer(source).scan()).parseProgram()
+            loader.loadImports(origin, program)
+            val script = Compiler(globalMutability).compile(program, returnLastExpr = true)
+            val result = vm.interpret(script)
+            if (result !is NilValue) println(displayString(result))
+        } catch (error: ModuleError) {
+            System.err.println(formatError(error.source, error.error))
+        } catch (error: LumenError) {
+            System.err.println(formatError(source, error))
+        }
+    }
+}