chore(idx): Add separator before type suffix in integer literals.

[daniel-wong-dfinity-org]

E.g. 42u64 is changed to 42_u64.

How to re-create these changes:

1. Build the tool. It is not in master yet; rather, it is in review in PR 9418. Until that happens, you'll first need to check out add-integer-type-suffix-separator-tool-daniel-wong. Then, bazel build //rs/tools/add-integer-type-suffix-separator.

2. Then, check out the master commit that this is based on (originally, 40ad82dbd8) and run the tool against all directories owned by the Internal Developer Experience team.

for DIRECTORY in \
    rs/prep \
    rs/tests/idx \
    rs/tests/testnets
do
    ./bazel-bin/rs/tools/add-integer-type-suffix-separator/add-integer-type-suffix-separator $DIRECTORY
done

[basvandijk]

As mentioned in #9418 it would be better to enforce this via clippy rather than built a custom tool.