Add precision specifier for %f, %g, %G string interpolation formats

[tetromino]

Now that Starlark has gained support for floating point, it makes sense to revisit the decision to not allow width or precision specifiers for % string interpolation formats.

This would be particularly useful for the java implementation of Starlark, where java.util.Formatter's default precision is 6 - meaning "%f" % 2" is currently "2.000000" instead of "2.0".

[brandjon]

My main concern is whether there are subtle differences possible in a naive implementation e.g. w.r.t. rounding behavior, and how much effort we have to go through to ensure compatibility with Python and across multiple Starlark implementations. But it sounds like a sensible addition.