fix: enable import of esm build

[bitjson]

Node's ESM import requires file extensions for all imports: https://nodejs.org/api/esm.html#mandatory-file-extensions. Without the .js extensions, a runtime error is thrown when cashscript is imported by esm projects.

This also updates electrum-cash, where some interfaces have changed (and if another package tries to share configuration with ElectrumNetworkProvider, they'll get type errors, as this one is more restrictive).

Finally, the explicit exporting of the network classes (in src/index.ts) solves an issue that esm dependents encounter: The requested module 'cashscript' does not provide an export named 'ElectrumNetworkProvider'.

(And if you're interested in migrating to ESM, hopefully this gets you a lot closer!)

[bitjson]

Ah, I also added 'import/extensions': ['error', { js: 'always' }], in .eslintrc.js; that makes it easier to notice when a rogue import would break esm imports at runtime.

[bitjson]

A project using this PR: https://gitlab.com/GeneralProtocols/anyhedge/library/-/merge_requests/193