feat: Allow Kong to exit with semantic exit codes (#507)

* feat: Allow Kong to exit with semantic exit codes

At Block, we've instrumented a number of commandline tools and set SLOs on some tools' reliability. To do that effectively, we had to partition usage errors from reliability issues. We looked at [prior art](https://github.com/square/exit?tab=readme-ov-file#reserved-codes-and-prior-art) and, taking inspiration from HTTP, defined [a set of semantic exit codes](https://github.com/square/exit?tab=readme-ov-file#about) in ranges: 80-99 for user errors, 100-119 for system errors.

We've been wrapping errors in `exit.Error` at whatever level of the stack can tell which class an error is and unwrapping them at exit (`os.Exit(exit.FromError(err))`).

This adds support for semantic exit codes to Kong, to `FatalIfErrorf`, which is used internally by `kong.Parse` and often used in Kong applications.

* feat: Exit 80 (Usage Error) when usage is syntactically or semantically invalid

* refactor: Always exit 80 (Usage Error) on a `ParseError` but don't wrap errors from hooks in `ParseError`

help_test.go

@@ -786,10 +786,11 @@ func TestUsageOnError(t *testing.T) {
 		Flag string `help:"A required flag." required`
 	}
 	w := &strings.Builder{}
+	exitCode := -1
 	p := mustNew(t, &cli,
 		kong.Writers(w, w),
 		kong.Description("Some description."),
-		kong.Exit(func(int) {}),
+		kong.Exit(func(code int) { exitCode = code }),
 		kong.UsageOnError(),
 	)
 	_, err := p.Parse([]string{})
@@ -806,6 +807,7 @@ Flags:
 test: error: missing flags: --flag=STRING
 `
 	assert.Equal(t, expected, w.String())
+	assert.Equal(t, 80, exitCode)
 }
 
 func TestShortUsageOnError(t *testing.T) {
@@ -813,10 +815,11 @@ func TestShortUsageOnError(t *testing.T) {
 		Flag string `help:"A required flag." required`
 	}
 	w := &strings.Builder{}
+	exitCode := -1
 	p := mustNew(t, &cli,
 		kong.Writers(w, w),
 		kong.Description("Some description."),
-		kong.Exit(func(int) {}),
+		kong.Exit(func(code int) { exitCode = code }),
 		kong.ShortUsageOnError(),
 	)
 	_, err := p.Parse([]string{})
@@ -829,6 +832,7 @@ Run "test --help" for more information.
 test: error: missing flags: --flag=STRING
 `
 	assert.Equal(t, expected, w.String())
+	assert.Equal(t, 80, exitCode)
 }
 
 func TestCustomShortUsageOnError(t *testing.T) {
@@ -840,10 +844,11 @@ func TestCustomShortUsageOnError(t *testing.T) {
 		fmt.Fprintln(ctx.Stdout, "🤷 wish I could help")
 		return nil
 	}
+	exitCode := -1
 	p := mustNew(t, &cli,
 		kong.Writers(w, w),
 		kong.Description("Some description."),
-		kong.Exit(func(int) {}),
+		kong.Exit(func(code int) { exitCode = code }),
 		kong.ShortHelp(shortHelp),
 		kong.ShortUsageOnError(),
 	)
@@ -856,4 +861,5 @@ func TestCustomShortUsageOnError(t *testing.T) {
 test: error: missing flags: --flag=STRING
 `
 	assert.Equal(t, expected, w.String())
+	assert.Equal(t, 80, exitCode)
 }