nim project create shouldn't offer languages that the backend doesn't support

[joshuaauerbachwatson]

Currently, there is a hard-coded list of languages in 'nim project create'. All languages in the list are accepted when specified, and all are listed in the usage help. There is no check of the backend's runtime repertoire, so, some of these languages may turn out to be unsupported at deploy time.

If there is a current API host defined, then

• at a minimum, any choice of language that is going to cause failure at deploy time should be rejected at project create time
• ideally, only languages that are going to work should be listed in the usage help (I forget whether it is feasible to compute that dynamically but I'll look into that)

If there is not a current API host defined, then this check cannot be carried out. There should probably be a warning that support for the chosen language may depend on what backend you end up deploying to.

[joshuaauerbachwatson]

Unfortunately, interrogating the controller is an asynchronous exchange, whereas the context of presenting usage help does not allow for asynchrony. So, the fix will only reject unsupported languages when you attempt to create. To lower the level of confusion, the languages will no longer be enumerated in the help. Instead, there is a generic statement in the extended description suggesting that the developer should use nim info --runtimes to see what is supported.

[joshuaauerbachwatson]

Fixed by PR #219