Ability to read dependencies.sh

[atakiya]

Should make integration to existing SS13 codebases quicker and easier.

Current plan is

if no `.byondversion` then
    read dependencies.sh
    write .byondversion

However which step should this happen on...

• Calling pin without arguments?
• Using a shimmed tool inside the directory? (possibly calling the tool executable to not bloat the shim)

And should this include an update command to read a new value, assuming only some contributors might use the tool in a project?

Sample data format of dependencies.sh:

#!/bin/sh

#Project dependencies file
#Final authority on what's required to fully build the project

# byond version
export BYOND_MAJOR=514
export BYOND_MINOR=1589

#rust_g git tag
export RUST_G_VERSION=1.2.0

#node version
export NODE_VERSION=14
export NODE_VERSION_PRECISE=14.16.1

# SpacemanDMM git tag
export SPACEMAN_DMM_VERSION=suite-1.7.3

# Python version for mapmerge and other tools
export PYTHON_VERSION=3.9.0