Undo ftplugin and indent settings when filetype changes

Set b:undo_ftplugin for matchpairs, matchit variables, b:syng_str and
the foldexpr settings, and b:undo_indent for indent settings, instead of
relying on whichever indent file was loaded last.

test/cases/undo.vue

@@ -0,0 +1,3 @@
+<template>
+  <div>x</div>
+</template>