improve text formatting (feature request)

Markdown is cool but too freaky for some people.
Making possible to use shortcuts could facilitate formatting a lot:
- CTRL+I _italic_
- CTRL+B **bold**
- CTRL+U underline
- CTRL+K [hyperlink](https://keybase.io/)
- CTRL+SHIFT+M monospace
- CTRL+SHIFT+S strikethrough
- etc.

This means that by pressing shortcut key the markdown formatting will be applied to the previously marked text. Writing Markdown manually slows dramatically down the writing tempo and makes Keybase uncomfortable to use which is something we probably do not want to.

Definitely also allow to use mouse for text formatting (e.g. small (most common) examples below the text editing window + after marking text also in right-click menu) as some people might not feel themselves comfortable by remembering those shortcut keys and probably not all formatting options might not be possible to apply shortcut keys. These shortcut keys could be also configurable through settings so there could be an option to adopt shortcut keys to your needs and probably change some as some keys might not be what some people are used to use.

Would propose to add small tabs in top of writing window similarly like here in GitHub but with the following names: **write | code**.

In _write_ mode you can write like usually with rich text formatting - this means that when marking text and pressing CTRL+B the marked text will be in bold and also shows so like in rich text editor. Basically a [WYSIWYG](https://en.wikipedia.org/wiki/WYSIWYG) mode.

In _code_ mode you can fine-tune the markdown manually if needed.

Here in GitHub you can only preview but not write in a human manner (real-time [WYSIWYG](https://en.wikipedia.org/wiki/WYSIWYG)) and still have to guess during writing, how the text look like as you don't probably have time to jump all the time between write and preview tabs - you would probably like to see immediately the formatting result while writing.

This applies to Keybase on all platforms and operating systems it is supported (including browser extensions).

Possibly also some other formatting, e.g. please see
- [Discord formatting](https://support.discordapp.com/hc/en-us/articles/210298617-Markdown-Text-101-Chat-Formatting-Bold-Italic-Underline-)
- [Slack formatting](https://get.slack.help/hc/en-us/articles/202288908-Format-your-messages)
- [Telegram formatting](https://www.techmesto.com/telegram-desktop-now-supports-text-formatting-using-keyboard-shortcuts/)
- [MS Teams formatting](https://docs.microsoft.com/en-us/microsoftteams/platform/concepts/bots/bots-text-formats), in additon [also this](https://support.office.com/en-us/article/format-a-message-in-teams-9f7c64e4-0316-472f-b1e8-430cebcfc1e5)
- etc.

The issue names no files, tests, or entry points and spans all supported platforms and browser extensions. Start by locating the message editor implementations, then clarify the scope before work begins; done would require an agreed implementation of the requested shortcuts, mouse formatting, and write/code modes.

Written by the indexing model from the issue text.