Print the environment variables that steer the V compiler and its tools.

Unlike `v doctor`, which is written for pasting into a bug report, the output
here is one `NAME="value"` line per setting, so it can be read by a script.
A variable that is not set reports the value V would use anyway, not an empty
string.

Usage:
  v env                 Print every reported setting.
  v env NAME            Print the value of a single setting, unquoted.
  v env -json           Print every reported setting as a JSON object.

Reported settings:
  VEXE                  the V executable that is running
  VROOT                 the V source tree that VEXE belongs to
  VOS                   the operating system V targets by default
  VARCH                 the architecture V targets by default
  VVERSION              the version and hash of the running compiler
  VMODULES              where vpm modules are installed and looked up
  VTMP                  the writable folder for temporary files
  V3CACHE               the base folder for the v3 module and object caches
  VTOOLS_CACHE_DIR      where compiled cmd/tools binaries are cached
  VCACHE                the object cache folder that `v wipe-cache` clears
  VFLAGS                extra flags applied to every V invocation
  VOSARGS               replaces the whole command line of every V invocation
  CC                    the C compiler, instead of the one V picks
  CFLAGS                extra flags for the C compiler
  LDFLAGS               extra flags for the C linker
  VJOBS                 how many parallel jobs V runs
  VERROR_PATHS          set to `absolute` to keep full paths in error messages
  VCOLORS               set to `always` or `never` to control colored output
  VCOVDIR               the folder coverage reports are written to
  VSTARTUP              a file the REPL runs at startup
  VQUIET                set to any value to silence the REPL

An unknown setting name is an error:

  v env NOSUCHVAR
  error: v env: unknown setting `NOSUCHVAR`.
  Known settings: VEXE VROOT VOS VARCH VVERSION VMODULES VTMP V3CACHE ...

Note that `VOSARGS` replaces every command line, so setting it in a shell makes
each later `v` invocation ignore its own arguments.
In the listing, values use JSON string escaping: quotes, backslashes and control
characters are escaped so each setting occupies one line. A named setting still
prints its raw value; -json returns the original values in a JSON object.
