When writing the JSON file, print the top-level keys (version numbers) in SemVer order (not lexicographical order)

[DilumAluthge]

Currently, we use JSON.jl's default sorting, which is to sort every dict in lexicographical order when printing. This is fine for e.g. the file_dict, but for the top-level keys (version numbers), this is less than optimal, because it means that the versions get printed in the order e.g. 1.0, 1.1, 1.10, 1.11, 1.12, 1.13, 1.2, 1.3, 1.4, ....

So, this PR turns off the default JSON.jl sorting, and then explicitly sorts things the way we want. For top-level keys (version numbers), we parse them as VersionNumbers and sort them in SemVer order. For other dicts, we manually do sort! to sort them in lexicographical order.

Other changes

I also included some smaller changes in this PR, let me know if you want me to split any of them out into their own separate PR:

• Do a full Pkg.update().
• Project.toml: Change the version number from 0.1.0 to 0.1.0-DEV.
• Project.toml: Add [compat] entries for all non-stdlib direct deps.
• Remove all uses of implicit imports; use explicit imports instead.
• Add a very basic Makefile so that we can just do make versions.json (just for convenience).

[DilumAluthge]

This is ready for review, but I want to do a full build locally to make sure it looks right.

[DilumAluthge]

I initially wanted to merge this separately from #67, but I think this PR and #67 are a little too closely linked, so I'm going to combine them.

Closing this PR, and I'll fold these changes into #67.