fix: error-strings custom funcs overwrites defaults

[alexandear]

The PR fixes a bug in the error-strings rule where configuring custom functions overwrites default functions.

When configuring custom functions to ["errors.Wrap"], the internal map r.errorFunctions would be:

	r.errorFunctions = map[string]map[string]struct{}{
		"fmt": {
			"Errorf": {},
		},
		"errors": {
			"Wrap": {},
		},
	}

but it should be:

	r.errorFunctions = map[string]map[string]struct{}{
		"fmt": {
			"Errorf": {},
		},
		"errors": {
			"Errorf":       {},
			"WithMessage":  {},
			"Wrap":         {},
			"New":          {},
			"WithMessagef": {},
			"Wrapf":        {},
		},
	}

Fixes #1243