expr: add support for =* and =~

[msteveb]

Adds glob match and regexp match operators to expr. e.g.

if {"abc" =* "a*"} {
  puts "matched glob" 
}
if {"abc" =~ "^a"} {
  puts "matched regexp" 
}

Is this useful enough to be included? Feedback welcome
(Needs documentation before merging)

[prpr19xx]

I don't think it's useless, so it must be (at least somewhat) useful, so why not include it?