feat(deno.json-importMap[deno add]): support inserting added dependencies into import map file when specified instead of deno.json

[HasanAlrimawi]

This PR serves as a starting point for addressing issue #24264 .

Flow
when using deno add "something", the program checks deno.json to see if it includes importMap key and if it does, the program adds the dependency there. However, if deno.json doesn't include the key, then the program adds the dependency under the imports key in deno.json.

Further work
If this feature is to be fully implemented using the approach in this PR, then the implementer should:

1. Check importMap file if it already includes the dependency to be added to avoid redundancy.
2. Handle mistyped/incorrectly-referenced importMap file.
3. Apply further testing on different scenarios, as this is a starting point solution.

[bartlomieju]

@dsherret can you please take a look at this one?

[dsherret]

@HasanAlrimawi sorry, but we're going to decline this one for now. See the reasoning in #24264 (comment)