[license] Improve license key encoding algorithm

Summary 💡

We could do better with the base64 encoding algorithm.

First, we can use the native ones, why do we even have a base64.ts file? It doesn't make sense to me when there is:

browser: window.btoa, window.atob
node.js: Buffer.from('Hello World!').toString('base64') and Buffer.from(b64Encoded, 'base64').toString()

available.

Second, if you open https://app-smtp.sendinblue.com/log#dc728631-08c2-4b7e-81d0-cee62d251e89 and search for "[MUI Store] Note added to your order from", you will see a load of license key with a leading = at the end. This is so painful. A double click doesn't select the whole key (macOS and Windows), but it's so long that it often overflow, so you don't see it:

the obvious fix is to replace = with _, quick win guaranteed.

Examples 🌈

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Motivation 🔦

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Order ID 💳 (optional)

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mui / mui-x