proposal: x/tools/cmd/stringer: add -trimsuffix

[golightlyb]

x/tools/cmd/stringer has -trimprefix option and should add a matching -trimsuffix option.

stringer's help would then look like:

Usage of stringer:
	stringer [flags] -type T [directory]
	stringer [flags] -type T files... # Must be a single package
For more information, see:
	https://pkg.go.dev/golang.org/x/tools/cmd/stringer
Flags:
  -linecomment
    	use line comment text as printed text when present
  -output string
    	output file name; default srcdir/<type>_string.go
  -tags string
    	comma-separated list of build tags to apply
  -trimprefix prefix
    	trim the prefix from the generated constant names
+  -trimsuffix suffix
+    	trim the suffix from the generated constant names
  -type string
    	comma-separated list of type names; must be set

stringer should exit with an error if the -trimprefix and -trimsuffix flags appear simultaneously, so that we don't need to think about an ordering like -trimprefix=ab -trimsuffix=bc for an input "abc". Also, giving a constant identifier both a prefix and a suffix is not something you ever see.

Rationale

From an unscientific code search, most Go "enum"-like constants take the form:

type Type int
const (
    Foo = Type(iota)
    Bar
    ...
)

or

type Type int
const (
    TypeFoo = Type(iota)
    TypeBar
    ...
)

However, less commonly but still not uncommonly, you see

type Type int
const (
    FooType = Type(iota)
    BarType
    ...
)

e.g. even in the stdlib (admittedly these don't have a String() method anyway):

• sort uses the "Hint" suffix
• Unicode uses the "Case" suffix
• http/cookie uses the "Mode" suffix
• flag uses the "OnError" suffix

Sometimes it's clearly more natural e.g. "FragmentShader" vs "ShaderFragment", "UpperCase" vs "CaseUpper".

There are advantages to using a prefix everywhere, but this is not to argue which is better, merely that the suffix form exists.

While there's probably some mileage in extracting the stringer into a library that can build a custom stringer command that can arbitrarily rewrite names, this is not that proposal. The stringer command itself should probably not be made any more complicated than this.