Add API to update import map in-place

[bartlomieju]

Need an API that can be used to update import map in-place after initial creation. This API will be used to inject mapping for built-in Node modules. The reason a special API is needed is that this injection is non-standard and will happen behind the scenes if users provide a CLI flag. The API must validate there are no entries already present in the import map that would clash with injected mappings.

Eg:

let mut node_mappings = HashMap::new();
node_mappings.insert("fs", "https://deno.land/std@0.110.0/node/fs.ts");
node_mappings.insert("net", "https://deno.land/std@0.110.0/node/net.ts");
...

let mut import_map = ImportMap::from_json(...)?;
import_map.update_in_place(node_mappings)?;  // <--- this will error out if any of the mappings is already in import map