Runscript 2.0 Beta 1

I'm not calling this a pre-release, simply because I want this to be the de facto version. Yes it's a beta, because I still haven't put everything I want into it yet, but it's usable and probably won't break too much between versions.

Changelog since v1.3.0:

• Complete rewrite of the script format, now uses TOML-like headers to denote scripts. Importantly, comments are consistently # now.
• Expanded shell language accepted; revamped executor which supports piping properly (thankfully).
• Changed the target/phase selection string to not include files. A specific file can be chosen with -f, otherwise it's target:phase
• Smaller number of built-in phases: only build, run, and test. However, phases can now be any [0-9A-Za-z_]+ string.
• What used to be default_positionals now no longer exists; just put a -- or : down instead.
• Variables are separated between shell variables and exported variables.
• A config file where you can change the default names of files run tries to execute, and the colours it uses for output.

Still to come:

• sourceing other shell scripts, for example, .env files.
• More configuration options, for example, to add special treatment for certain phases
• More syntax, such as for metadata on certain phases, commands that only run on a particular platform, prerequisite phases...