feat(core): add error types and source reporting

src/main/kotlin/lumen/LumenError.kt

@@ -0,0 +1,49 @@
+package lumen
+
+open class LumenError(
+    val rawMessage: String,
+    val line: Int,
+    val column: Int = 0
+) : RuntimeException(
+    buildString {
+        if (line > 0) {
+            append("[line ").append(line)
+            if (column > 0) append(", column ").append(column)
+            append("] ")
+        }
+        append(rawMessage)
+    }
+)
+
+class LexError(message: String, line: Int, column: Int = 0) : LumenError(message, line, column)
+
+class ParseError(message: String, line: Int, column: Int = 0) : LumenError(message, line, column)
+
+class CompileError(message: String, line: Int, column: Int = 0) : LumenError(message, line, column)
+
+class RuntimeError(message: String, line: Int, column: Int = 0) : LumenError(message, line, column)
+
+fun formatError(source: String, error: LumenError): String {
+    val header = "error: ${error.rawMessage}"
+    if (error.line <= 0) return header
+    val lines = source.split("\n")
+    if (error.line > lines.size) {
+        return "$header\n --> line ${error.line}"
+    }
+    val sourceLine = lines[error.line - 1]
+    val gutter = error.line.toString()
+    val pad = " ".repeat(gutter.length)
+    val builder = StringBuilder()
+    builder.append(header).append("\n")
+    val position = if (error.column > 0) "line ${error.line}, column ${error.column}" else "line ${error.line}"
+    builder.append(pad).append(" --> ").append(position).append("\n")
+    builder.append(pad).append(" |\n")
+    builder.append(gutter).append(" | ").append(sourceLine).append("\n")
+    if (error.column > 0) {
+        val caretPad = " ".repeat((error.column - 1).coerceAtLeast(0))
+        builder.append(pad).append(" | ").append(caretPad).append("^")
+    } else {
+        builder.append(pad).append(" |")
+    }
+    return builder.toString()
+}