runtime: Add [ft]plugin/shada.vim files that automatically open .shada

runtime/ftplugin/shada.vim

@@ -0,0 +1,18 @@
+if exists('b:did_ftplugin')
+  finish
+endif
+
+let b:did_ftplugin = 1
+
+function! ShaDaIndent(lnum)
+  if a:lnum == 1 || getline(a:lnum) =~# '\mwith timestamp.*:$'
+    return 0
+  else
+    return shiftwidth()
+  endif
+endfunction
+
+setlocal expandtab tabstop=2 softtabstop=2 shiftwidth=2
+setlocal indentexpr=ShaDaIndent(v:lnum) indentkeys=<:>,o,O
+
+let b:undo_ftplugin = 'setlocal et< ts< sts< sw< indentexpr< indentkeys<'