[Rd] Underscores in package names

[Jim Hester]

Are there technical reasons that package names cannot be snake case?
This seems to be enforced by `.standard_regexps()$valid_package_name`
which currently returns

   "[[:alpha:]][[:alnum:].]*[[:alnum:]]"

Is there any technical reason this couldn't be altered to accept `_`
as well, e.g.

  "[[:alpha:]][[:alnum:]._]*[[:alnum:]]"

I realize that historically `_` has not always been valid in variable
names, but this has now been acceptable for 15+ years (since R 1.9.0 I
believe). Might we also allow underscores for package names?

Jim