Add matching pair insertion to markdown textarea

[silverwind]

1. Our textarea already has some editor-like feature like tab indentation, so I thought why not also add insertion of matching closing quotes/brackets over selected text. This does that.
2. textareaInsertText is replaced with replaceTextareaSelection which does the same but create a new edit history entry in the textarea so CTRL-Z works. The button that inserts tables into the textarea can now also be reverted via CTRL-Z, which was not possible before.

[silverwind]

Backtick added as well. There is a catch with that key because it's a dead key on some keyboard and MDN specifies:

If the KeyboardEvent represents the press of a dead key, the key value must be "Dead".

So backtick will only work on keyboard layouts where it is not a dead key (e.g. NoDeadKeys layout).