The ability to split a long string over multiple lines

When using a really long string like `https://mystorageaccount.blob.core.windows.net/system/Microsoft.Compute/Images/windows-2016-datacenter/packer-osDisk.8c22742f-d22d-4f1a-babd-7712381c413e.vhd`, it forces editors to horizontally scroll. I'd like the ability to split the definition of strings like this over multiple lines. Note that this is different than splitting text over multiple lines with:

```
text = <

Start by reviewing HCL's existing string syntax and the discussion in this issue, including how the requested multiline form should differ from heredocs. Define the accepted syntax and its parsing and formatting behavior before identifying the relevant implementation and tests; done means the syntax is specified and covered by tests.

Written by the indexing model from the issue text.