On enabling post-MVP features by default

In a recent Binaryen PR the question of when to enable post-MVP features in tooling came up. In particular, since the mutable-global proposal had been finished meanwhile, the suggestion there was to enable the feature by default in Binaryen (and have a Default feature set to target finished features). On the AssemblyScript side, a good strategy seems to be to enable finished proposals by default as soon as our toolchain supports them in a stable manner, closely aligning with the standardization process. Until there is some sort of feature detection this also means: If a targeted engine supports more than the finished proposals, those must be explicitly enabled in tooling. Likewise, if a targeted engine supports less than the finished proposals, those must be explicitly disabled in tooling.

As suggested by @kripken I'm opening this issue so we can coordinate.

Start by reading the linked Binaryen PR and the finished-proposals list to understand the proposed default-feature policy. Then review how AssemblyScript and related tooling currently coordinate feature support; done means reaching and documenting an agreed policy for enabling finished proposals and handling engines with different support levels.