ssr: support regex-style backreferences to previously used expressions

In some cases, just like in regex, it is needed to reference an expression previously declared:

rust-analyzer ssr \
  'while $i < $end { $block; $i = $i.wrapping_add(1); } ==>> for $i in 0..$end { $block; }'

To match this type of code:

while counter < 100 {
  // ...
  counter = counter.wrapping_add(1);
}

to transform it into

for counter in 0..100 {
  // ...
}

Currently I get an error:

can't parse `rule`, Parse error: Placeholder `$i` repeats more than once

Start with the rust-analyzer SSR command, reproducing the shown rule and its “Placeholder $i repeats more than once” error. Trace how SSR rules are parsed and how placeholders are matched; done means the example rule accepts the repeated expression and transforms the shown while loop into the for loop.