runtime(rust): use textwidth=100 for the Rust recommended style

runtime(rust): use textwidth=100 for the Rust recommended style

The help text here said 99 was the recommended style for the standard
library, but I can't find a citation for this anywhere. In contrast the
Rust Style Guide hosted on rust-lang.org
[says](https://doc.rust-lang.org/stable/style-guide/#indentation-and-line-width)
the maximum line width is 100, and rustfmt
[agrees](https://github.com/rust-lang/rust/blob/37aa2135b5d0936bd13aa699d941aaa94fbaa645/src/tools/rustfmt/src/config/options.rs#L570).

Having the two disagree causes an annoying off-by-one error in vim: if
you configure vim to highlight too-long lines then it will occasionally
complain about a line that rustfmt refuses to fix.

closes: #18892

Signed-off-by: Aaron Jacobs <jacobsa@google.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>