Add environment variable help under --envs-help & refactor environment variable usage

[Gedochao]

Changes are as follows:

• refactors the code to have all environment variables we use listed as constants in one spot (which makes it easier to track)
  • this doesn't include all of the envs used by our dependencies, like Bloop or Coursier, so it's not meant to be truly exhaustive from the user's perspectvie;
• limits environment variables saved for bsp in setup-ide to only those that are necessary;
• adds environment variable dedicated help under --envs-help, which lists all of the envs we use with short descriptions:

scala-cli --env-help --power
# The following is the list of environment variables used and recognized by Scala CLI.
# It should by no means be treated as an exhaustive list
# Some tools and libraries Scala CLI integrates with may have their own, which may or may not be listed here.
# 
# Scala CLI
#   SCALA_CLI_CONFIG              Scala CLI configuration file path
#   SCALA_CLI_HOME                Scala CLI home directory
#   SCALA_CLI_INTERACTIVE         Interactive mode toggle
#   SCALA_CLI_INTERACTIVE_INPUTS  Interactive mode inputs
#   SCALA_CLI_POWER               Power mode toggle
#   SCALA_CLI_PRINT_STACK_TRACES  Print stack traces toggle
#   SCALA_CLI_SODIUM_JNI_ALLOW    Allow to load libsodiumjni
#   SCALA_CLI_VENDORED_ZIS        Toggle io.github.scala_cli.zip.ZipInputStream
# 
# Java
#   JAVA_HOME                     Java installation directory
#   JAVA_OPTS                     Java options
#   JDK_JAVA_OPTIONS              JDK Java options
# 
# Coursier
#   COURSIER_CACHE                Coursier cache location
#   COURSIER_MODE                 Coursier mode (can be set to 'offline')
# 
# Spark
#   SPARK_HOME                    (power) Spark installation directory
# 
# Miscellaneous
#   PATH                          The app path variable
#   DYLD_LIBRARY_PATH             Runtime library paths on Mac OS X
#   LD_LIBRARY_PATH               Runtime library paths on Linux
#   PATHEXT                       Executable file extensions on Windows
#   SHELL                         The currently used shell
#   VCVARSALL                     Visual C++ Redistributable Runtimes
#   ZDOTDIR                       Zsh configuration directory
# 
# Internal
#   CI                            (power) Marker for running on the CI