chore: reworking CLI + import specifiers

[harrysolovay]

Resolved #258

This PR includes a variety of tweaks to the CLI:

• specify target chain via SCALE or chainspec file paths or WebSocket URL.
• choose the Capi import specifier of codegened output (denoland/x is the default). This enables us to override to a local version or the NPM name.

Usage is slightly different.

deno run -A -r https://deno.land/x/capi/codegen.ts \
  --src=wss://rpc.polkadot.io \
  --out=polkadot \
  --import=capi

In the context of this repo, we could run the following:

deno task run codegen.ts \
  --src=wss://rpc.polkadot.io \
  --out=polkadot \
  --import=mod.ts

Given that we specify mod.ts as the Capi import specifier, the files generated into the polkadot dir will import capi via ../mod.ts.