Apply % tooltip to pie with stack: 'normalize'

[kanitw]

For normalized stacked charts (like bar), we automatically generate tooltips that calculates %.

The same could be applied to pie charts with stack: normalize too.

Essentially, the second block if (stack && stack.fieldChannel === channel && stack.offset === 'normalize') { in the code below in src/compile/mark/encode/tooltip.ts should be applicable to theta & radius too, not just x and y.