`text_with_carats` panics and has no unit tests

[rbalicki2]

• text_with_carats panics on line 73. It should not.
• To repro, modify Pet.PetDetailRoute to be:

  field Query.PetDetailRoute($id: ID!) @component {
    pet(id: 1) {
      __typename
    }
    pet(id: $id) {
      PetTaglineCard
      name
      id
      bar: id
      baz: id
    }
  }

then run pnpm watch-pet-demo:

pnpm watch-pet-demo

> @ watch-pet-demo /Users/rbalicki/code/isograph
> ./target/debug/isograph_cli --config ./demos/pet-demo/isograph.config.json --watch

Starting to compile.
Error when compiling.

thread 'main' panicked at crates/common_lang_types/src/text_with_carats.rs:73:40:
attempt to subtract with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 ELIFECYCLE  Command failed with exit code 101.

Uncovered when investigating issue #24