clippy shuld check clippy::string_slice

[toyamagu-2021]

Please explain the motivation behind the feature request.
Does this feature solve a particular problem you have been experiencing? What opportunities or use cases would be unlocked with this feature?

• Splitting a string by an array is not safe.
  • fix: use safe_truncate to truncate charactor (#3263) #3264
  • bug: String truncation fails with "byte index is not a char boundary" for non-ASCII text #3263

Describe the solution you'd like
A clear and concise description of what you want to happen.

• add clippy::string_slice for lint
  • https://rust-lang.github.io/rust-clippy/master/index.html#string_slice

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

• I have verified this does not duplicate an existing feature request