toolchain: add new debugging tool using magenta_debug

toolchain/mxdbg/client/line-ed/buffer.c

@@ -0,0 +1,44 @@
+#include "buffer.h"
+
+#include "line-ed.h"
+
+#include <fx/stringstream.h>
+#include <fx/wstr.h>
+
+const fx_wchar *line_start(struct line_ed *ed, size_t y)
+{
+	const fx_wchar *line = ed->l_buf;
+
+	for (size_t i = 0; i < y; i++) {
+		line += fx_wstrcspn(line, L"\n");
+
+		if (*line == '\n') {
+			line++;
+		}
+	}
+
+	return line;
+}
+
+size_t line_length(struct line_ed *ed, size_t y)
+{
+	const fx_wchar *line = ed->l_buf;
+
+	for (size_t i = 0; i < y; i++) {
+		line += fx_wstrcspn(line, L"\n");
+		if (*line == '\n') {
+			line++;
+		}
+	}
+
+	if (*line == '\0') {
+		return 0;
+	}
+
+	size_t len = fx_wstrcspn(line, L"\n");
+	if (line[len] == '\n') {
+		len++;
+	}
+
+	return len;
+}