Melange support

[actionshrimp]

Simpler setup after learning from migrating other bits and pieces over to use melange. Supercedes #64.

• The decoders lib is available asdecoders for both 'native' builds, and melange builds. That way the decoders interface can be used in shared code.
• The actual browser implementation of decoders is available as melange-decoders.

Caveats:

• The bucklescript build still works, but Bs_json and Bs_xml modules are renamed to Js_json and Js_xml which is a breaking API change.
• The whole repo needs (using melange 0.1) which requires dune >= 3.8.
• As dune 3.8 isn't available on the windows opam-repository overlay currently, the windows build is broken on CI.
• The decoders package now requires melange to be installed.

To avoid forcing all ocaml-decoders users, even those who don't care about melange, to upgrade to dune 3.8 and install melange, we could either:

• leave this as a melange branch which can be pinned by anyone who needs melange-decoders, as they will be using dune 3.8 anyway.
• just merge it and anyone who needs an older version of dune or melangeless setup can stay on an older version of decoders as decoders hasn't changed much for a while.

This also hopefully resolves https://github.com/mattjbray/ocaml-decoders/issues/59 - anyone wanting to use melange-decoders can do:

opam pin decoders git+https://github.com/mattjbray/ocaml-decoders.git#melange
opam pin melange-decoders git+https://github.com/mattjbray/ocaml-decoders.git#melange

to access the melange supported version.

[actionshrimp]

This all seems to be working ok as per the (updated) description now.