Insert pipe character | when editing multiline strings with stringMargin

[olafurpg]

Is your feature request related to a problem? Please describe.

When editing multiline strings with .stripMargin it's currently necessary to manually type the | character and indent it.

Describe the solution you'd like

It would be nice if Metals inserted the | character at the correct indentation as we type, the same way it works in IntelliJ.

Describe alternatives you've considered

In the PR scalameta/metals-vscode#121, we added basic support for similar functionality in the VS Code extension. This solution has several limitations however:

• it triggers even when editing outside multiline strings

• it doesn't auto-insert | for copy-pasted multiline code

• it only works for VS Code

Additional context

In LSP there is a textDocument/formattingOnType request that allows us to produce the same behaviour that allows us to implement this analysis on top of tokens and make it work for all editors.

[mzarnowski]

partially fixed by #865

What is still missing is inserting "|" when copy-pasting inside a multiline strings