Add 'only-explicit-rules' option to display explicit grammar rules

[ydah]

This PR, add 'only-explicit-rules' option to display explicit grammar rules.

--trace=rules:

❯ exe/lrama --trace=rules ./spec/fixtures/common/basic.y
Grammar rules:
$accept -> program EOI
program -> class
program -> '+' strings_1
program -> '-' strings_2
class -> keyword_class tSTRING keyword_end
$@1 -> ε
$@2 -> ε
class -> keyword_class $@1 tSTRING '!' keyword_end $@2
$@3 -> ε
$@4 -> ε
class -> keyword_class $@3 tSTRING '?' keyword_end $@4
strings_1 -> string_1
strings_2 -> string_1
strings_2 -> string_2
string_1 -> string
string_2 -> string '+'
string -> tSTRING
unused -> tNUMBER

--trace=only-explicit-rules:

❯ exe/lrama --trace=only-explicit-rules ./spec/fixtures/common/basic.y
Grammar rules:
$accept -> program EOI
program -> class
program -> '+' strings_1
program -> '-' strings_2
class -> keyword_class tSTRING keyword_end
class -> keyword_class tSTRING '!' keyword_end
class -> keyword_class tSTRING '?' keyword_end
strings_1 -> string_1
strings_2 -> string_1
strings_2 -> string_2
string_1 -> string
string_2 -> string '+'
string -> tSTRING
unused -> tNUMBER

[yui-knk]

I think trace is used for debug in such case it's beneficial to render information as far as possible.
So that I want to keep to render mid-rule actions too.

[ydah]

Thank you. I certainly agree with outputting as much information as possible in trace.

I would be happy to see only the grammar rules and exclude actions, when I research structure of grammar rule. What do you think about adding an option to trace, for example, like only_rules?

[yui-knk]

I agree to add new option to --trace, what do you think about only-rules (not _ but -)?

[Added]
The rules for mid-rule actions, like $@1 -> ε, are also rules which parser generator internally creates.
So that I prefer only-explicit-rules or something like that.

[ydah]

@yui-knk Thank you for your suggestion! The only-explicit-rules option has been added to --trace.