Add conditional Suggest

[tdakkota]

Conditional suggesting may be useful to simplify rules and to reduce some matching boilerplate in similar rules.
Also, it makes quickfix specialization very easy.

API

// SuggestIf assigns a quickfix suggestion for the matched code if given cond is satisfied.
func (m Matcher) SuggestIf(cond bool, suggestion string) Matcher {

Example of reducing boilerplate

From

    isSigned := func(v dsl.MatcherVar) bool {
      return v.Type.Is(`int`) || v.Type.Is(`int8`) || v.Type.Is(`int16`) || v.Type.Is(`int32`)  || v.Type.Is(`int64`) 
    }
    m.Match(`fmt.Sprintf("%d", $arg)`, `fmt.Sprintf("%v", $arg)`).
      Where(m["arg"].Type.Is("int64")).
      Suggest(`strconv.FormatInt($arg, 10)`)
    m.Match(`fmt.Sprintf("%d", $arg)`, `fmt.Sprintf("%v", $arg)`).
      Where(m["arg"].Type.Is("int")).
      Suggest(`strconv.Itoa($arg)`)
    m.Match(`fmt.Sprintf("%d", $arg)`, `fmt.Sprintf("%v", $arg)`).
      Where(
          !m["arg"].Type.Is("int") &&
          !m["arg"].Type.Is("int64") &&
          isSigned(m["arg"]) ,
      ).
      Suggest(`strconv.FormatInt(int64($arg), 10)`)

to

        isSigned := func(v dsl.MatcherVar) bool {
           return v.Type.Is(`int`) || v.Type.Is(`int8`) || v.Type.Is(`int16`) || v.Type.Is(`int32`)  || v.Type.Is(`int64`) 
        }
	m.Match(
		`fmt.Sprintf("%d", $arg)`,
		`fmt.Sprintf("%v", $arg)`,
	).Where(isSigned(m["arg"])).
		// Suggest Itoa if arg has 'int' type
		SuggestIf(m["arg"].Type.Is("int"), `strconv.Itoa`).
		// Suggest FormatInt without casting if arg has 'int64' type
		SuggestIf(m["arg"].Type.Is("int64"), `strconv.FormatInt($arg, 10)`).
		// Otherwise, suggest FormatInt with cast
		Suggest(`strconv.FormatInt(int64($arg), 10)`)

[quasilyte]

I'll experiment with #302 and we can see whether it helps here or not.