Create all_possible_subsequences.js

[100mya]

javascript code for all possible subsequences using backtracking

[appgurueu]

Side effects (console.log) are a poor API for this. Ideally this should be a generator for much better reusability and testability. A callback or pushing to an array may also be acceptable (though I'd prefer the generator).

Needs proper tests rather than an example in the file.

Also seems to be partially redundant with Recursive/SubsequenceRecursive.js, which solves a more specific problem (strings, only nonempty subsequences). Ideally SubsequenceRecursive.js should be rewritten in terms of this.