Project-local compiler versions with .vvmrc

If a project contains a .vvmrc file, commands like v run ., v run file.v, or v file.v will try to find and delegate to the requested V compiler version. This makes it easier to keep a project pinned to a known compiler release.

The file should contain one version per project, for example:

0.4.12

Both 0.4.12 and v0.4.12 are accepted. The special aliases latest and current keep using the currently running compiler.

V searches for .vvmrc from the target path upward and stops at repository and project boundaries such as .git, .hg, .svn, and .v.mod.stop.