Add stringToName special case for all-underscores identifier strings

[wandernauta]

Before, parsing the following PVL program like the following would give an assertion error, since it would attempt to build a preferred name with no parts:

void ___() {
}

It now gets a preferred name with one part, the string "___".

Also adds a unit test for stringToName.

[superaxander]

Looks like the compilation fails because github updated the ubuntu-latest runner image from ubuntu-22.04 to ubuntu-24.04 which contains a newer version of LLVM while VCLLM still expects LLVM 15 to be present. Once #1159 is merged (hopefully this week) this will be resolved.

[wandernauta]

Indeed. I'll rebase this (and #1261) on dev once that PR is in. It looks really cool!