PR Summary

Fixes #2337
Fixes #13204
There were a lot of different ideas posted in this thread, and this is just my own personal preference/idea on how to solve this.
I am open to other ideas if that's what the team/community wants.

You can add as many @ signs as you want to the here-string, if you add 2 or more @ signs you allow the string terminator to be indented like this:

Write-Host @@'
    Hello world!

        Here-strings are cool
    '@@

The line with the fewest leading whitespaces (not including empty lines) dictate how many whitespace characters are removed from each line so in the previous example the first line would have no spaces, and the third would have 4 spaces.

Single line here-strings are also supported: Write-Host @'Hello world!'@ like the multi-line variant you can add as many @ signs as you want.

There are 3 reasons why I picked multiple @ signs over multiple ' .
1: It's easier to visually easier to see/count if you write @@@@@' VS @''''' especially when you also consider double quotes and all the different single/double quote variants that are also supported.
2: If you decide to swap between a single quote and double quote here-string it's easier to just change 1 character in each end instead of multiple characters in each end.
3: In the case of single line strings it allows the string to start with any character. It quotes were used you wouldn't be able to start the string with a quote character because it would be counted as part of the "header" section of the here-string.

PR Context

It's annoying to not be able to match the surrounding indentation if you for example use a herestring inside a function. Even if the content doesn't care about the leading whitespace the string terminator does so you end up with an ugly little '@ if you try to indent all the code that can be indented.
Sometimes when writing string filters you end up needing to quote values inside the string which means you either need to remember to escape or use the opposite quotes. It's nicer to just start a filter with @' or @" and no longer worry about having to escape characters inside the string.

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