Rollup merge of #136749 - mzeitlin11:extend-asciichar, r=scottmcm

Implement Extend<AsciiChar> for String

Implement `Extend<AsciiChar>` for `String` as suggested in rust-lang/rust#110998 (comment). Also implements `Extend<&AsciiChar>` since there's an analogous impl for `Extend<&char>`, but happy to remove if not thought useful.

r? `@scottmcm`
since you requested it, but no pressure to review!