Rebrand polkadot{.js} to Polkadot Developer Signer

[TarikGul]

In line with the following blog post - we are aiming to rebrand the polkadot{.js} extension to be the Polkadot Developer Signer.

There is currently one final issue I need to resolve in this PR which involves the EXTENSION_PREFIX. The one main issue with the EXTENSION_PREFIX is the short_name that is used in the manifest may be used as a backup for the prefix. I want to make sure there is no friction and lose of data if the below is changed. If there might be, I say we leave the short_name to polkadot{.js}, and ensure only UI branding changes and internal things stay the same.

extension/packages/extension-base/src/defaults.ts

Line 5 in 9447ce2

export const EXTENSION_PREFIX = process.env['EXTENSION_PREFIX'] || '';

extension/packages/extension-base/src/stores/Accounts.ts

Lines 12 to 16 in 9447ce2

	EXTENSION_PREFIX && EXTENSION_PREFIX !== 'polkadot{.js}'
	? `${EXTENSION_PREFIX}accounts`
	: null
	);
	}