Longer quoted strings

**I propose we** add longer quoted strings (7+ quotes since 6 quotes specify an empty triple-quoted string) for when we need to embed source code that contains triple quoted strings. It is in line with C#'s design of [raw string literals with variable length of starting quotes](https://github.com/dotnet/csharplang/blob/345fa10e4f19b5510b60cbc4822ad953213a906f/proposals/csharp-11.0/raw-string-literal.md). Here, we can follow C#'s design of the multiline literal with the end delimiter specifying the indentation size (must be whitespaces, not any other character):

```fs
let fsharpCode = """""""
let string = $"""{String.replicate 3 "Hello World!\n"}"""
"""""""
let fsharpCode' = """""""
let string = $"""{String.replicate 3 "Hello World!\n"}"""
"""""""
fsharpCode = fsharpCode' // true
```

The newlines directly next to the delimiters will not be included in the string. Therefore, the string has the following content without preceding or succeeding newlines:
```fs
let string = $"""{String.replicate 3 "Hello World!\n"}"""
```

Since 7 quotes are already quite long, we won't allow single-line versions of it. So these are illegal:

```fs
let fsharpCode = """""""let string = $"""{String.replicate 3 "Hello World!\n"}""" """"""" // error - no single line version
let fsharpCode' = """""""
let string = $"""{String.replicate 3 "Hello World!\n"}""" """"""" // error - ending delimiter must be on its own line
let fsharpCode'' = """""""let
string = $"""{String.replicate 3 "Hello World!\n"}"""
""""""" // error - starting delimiter must be followed by a newline
```

The minimum lines that a longer quoted string must occupy is 3:
```fs
let empty = """""""
""""""" // error - too short - leave some space for adding content!
let empty' = """""""

""""""" // ok
```
This is because the starting delimiter requires a newline after, and the ending delimiter requires a newline before.

Interpolation will work:
```fs
let fsharpCode = $$"""""""
let string = $"""{String.replicate %%d{{3}} "Hello World!\n"}"""
"""""""
```
and produces the same string as before.
**The existing way of approaching this problem in F# is** to use runtime string manipulation which is less visually appealing and costs performance.

## Pros and Cons

**The advantages of making this adjustment to F# are**
1. Alignment with C#
2. Convenience
3. Visual appeal compared to triple quoted strings

**The disadvantage of making this adjustment to F# is** the learning complexity and implementation and maintenance costs.

## Extra information

**Estimated cost (XS, S, M, L, XL, XXL):** S to M

**Related suggestions:**
- https://github.com/dotnet/csharplang/issues/4304

## Affidavit (please submit!)

Please tick these items by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [StackOverflow](http://stackoverflow.com)) and I have searched StackOverflow for discussions of this issue
* [x] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to [the compiler and tooling repository](https://github.com/dotnet/fsharp)
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] I or my company would be willing to help implement and/or test this

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.

Start by reviewing the proposed multiline examples and the related C# raw string literal issue linked in the report. Check the delimiter, indentation, interpolation, newline, and minimum-length rules described here; done means the examples behave as specified, including the listed invalid forms being rejected.

Written by the indexing model from the issue text.