command/format: multi-line rendering for unchanged strings

[apparentlymart]

We have a special treatment for multi-line strings that are being updated in-place where we show them across multiple lines in the plan output, but we didn't use that same treatment for rendering multi-line strings in isolation such as when they are being added for the first time.

Here we detect when we're rendering a multi-line string in a no-change situation and render it using the diff renderer instead, using the same value for old and new and thus producing a multi-line result without any diff markers at all.

This improves consistency between the change and no-change cases, and makes multi-line strings (such as YAML in block mode) readable in all cases.

Before:

  # null_resource.foo will be created
  + resource "null_resource" "foo" {
      + id       = (known after apply)
      + triggers = {
          + "a" = "foo\nbar\nbaz\n"
        }
    }

After:

  # null_resource.foo will be created
  + resource "null_resource" "foo" {
      + id       = (known after apply)
      + triggers = {
          + "a" = <<~EOT
                foo
                bar
                baz
            EOT
        }
    }

This fixes #21817 as closely as we can within our existing diff heuristics. Specifically, the new treatment of the list-of-multiline-strings case is improved but still not ideal because the list diffing algorithm only ever produces additions and removals and never in-place updates, so a list like the values in that issue's example would use the above multi-line rendering for both the old and new values but would not interleave them into a single line-oriented diff because it doesn't understand that the new element zero is intended as an in-place edit to the old element zero, rather than a replacement. Maybe we'll improve on that further someday, but for now just showing the lines of the string in full should hopefully be a reasonable compromise.

[ghost]

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