Add a string function that dedents multi-line strings

[fmeum]

The usability of multi-line string literals could be greatly improved if there was a way to have leading and trailing newlines stripped and common indent removed, similar to how Java textblocks work. Python doesn't do this, but offers functions such as inspect.cleandoc and textwrap.dedent that perform such postprocessing. It would be very convenient for Starlark strings to provide such a function as a member function.

Before:

def f():
    return """\
foo
bar
baz"""

After:

def f():
    return """
        foo
        bar
        baz
        """.dedented()

Implementations could detect obvious calls to detended() and optimize them so that there is a single dedented string instance.