Ensure ggproto methods start with {

[yjunechoe]

As discussed in #6459, this PR changes instances of function() ... into function() { ... } for ggproto methods.

There was one not-so-trivial case of ScaleDiscrete$transform:

ggplot2/R/scale-.R

Lines 981 to 983 in 86b32ce

	transform = identity,

Because base::identity() is a one-liner without {, this fails the test:

identity
#> function (x) 
#> x
#> <bytecode: 0x000001b6a4393740>
#> <environment: namespace:base>

In this PR, ScaleDiscrete$transform is now the spelled-out version:

  transform = function(self, x) {
    x
  },