Make command templates context type aware

[bwateratmsft]

Fixes #2168. Template commands are made context-aware, so that different commands can match different contexts.

With the old behavior, the order is as follows:

1. Look for any template with a match property that matches
2. Look for any template without a match property
3. Use the default template

With this new behavior, the order is as follows (more or less the same as above):

1. Look for any template whose context type matches AND with a match property that matches
2. Look for any template whose context type matches AND without a match property
3. Use the default template whose context type matches

If no contextType property is explicitly specified on any template, it is treated as 'all' and will apply in any context. 'legacy' currently matches only 'moby' contexts, and 'new' currently matches only 'aci' contexts (though it is likely there will be more in the future).

TODO: create an issue or PR to update the docs.

[bwateratmsft]

@karolz-ms and I discussed an alternative approach to new / aci / legacy / all:

1. Only aci and moby would be defined, right now (same as what is defined for ContextType).
2. An undefined contextType value would be equivalent to "all". Today, it would mean moby or aci, but if a new context type abc appears, it would still match this and work for it.
3. The existing commands except compose up/down would have contextType undefined (as far as we know they work in all contexts).
4. The new compose up/down would have contextType undefined to match any new context, and the old compose up/down would have contextType == moby.
5. Context type could be defined as an array which would be treated as an OR list, i.e. ["moby", "aci"] would mean either moby or aci (today, this is all possible values, but that may not be true in the future). For example, if a new context type abc appears, then a command with contextType == ["moby", "aci"] would not match it, but one with contextType == undefined would match it. To make such a command match with all three you could do ["moby", "aci", "abc"] (or leave it undefined, of course).
6. Template preference would go similar to the existing match property, preferring, in order:
   1. Settings-defined templates with either match or contextType, where all specified properties match.
   2. Settings-defined templates with neither match nor contextType.
   3. Hard-coded default templates (only one should ultimately match, to avoid prompting).

[karolz-ms]

Nice