Improve config ovewrite

[tomusdrw]

Currently it's possible to overwrite some part of the config using the pseudo-jq syntax (introduced in #733).

So I can just load a different chain spec, like so:

npm start -- --config=default --config=".chain_spec = ./chain-spec.json"

However it's necessary to provide the initial --config=default as a base config, otherwise we get a rather cryptic error:

> NODE_ENV=development tsx ./index.ts --config=.chain_spec = ./chain-spec.json import ./fallback.bin

LOG   [config] 🔧 Loading config
LOG   [config] 🔧 Applying '.chain_spec = ./chain-spec.json'
Error: Unable to parse config: Error: [<root>] Error while parsing the value: Error: [<root>] Unexpected or missing keys: <missing>,"$schema" | <missing>,"authorship" | <missing>,"flavor" | <missing>,"version"
          Data: chain_spec,database_base_path
          Schema: $schema,version,flavor,chain_spec,database_base_path,authorship

My proposed solution would be to simply always apply default config at the very beginning. However we should make sure that loading another full config will ovewrite the entire config instead of doing a deep merge, so: --config=default --config=dev should simply load dev config and not deep merge default with dev (as it is currently)

CC @skoszuta