runtime(arduino): Add Arduino ftplugin and indent files (#14811)

We already have Arduino syntax file, but we didn't have ftplugin and
indent files.

This commit adds a basic ftplugin file and a basic indent file.
Both of them are derived from {ftplugin,indent}/c.vim.

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/indent/arduino.vim

@@ -0,0 +1,16 @@
+" Vim indent file
+" Language:	Arduino
+" Maintainer:	The Vim Project <https://github.com/vim/vim>
+"		Ken Takata <https://github.com/k-takata>
+" Last Change:	2024 Apr 03
+
+" Only load this indent file when no other was loaded.
+if exists("b:did_indent")
+  finish
+endif
+let b:did_indent = 1
+
+" Use C indenting.
+setlocal cindent
+
+let b:undo_indent = "setl cin<"