Add visit to make MessageReference and Term recognize VariableReference

[andrew000]

Term recognition

Term may exist in .ftl and FTL-Extract will not touch them. Be careful when creating a Term, as a syntax error in it will return an error FTLExtractorCantFindTermError.
If using Term with common Message, Term should contain all VariableReferences required by Message, or Message will be commented.

-selector =
    { $lang ->
        [uk] { $lang }, Ok
       *[other] { $lang }, 🤔
    }
test-message = Some text { -selector }

async def test(i18n: Any) -> None:
    i18n.test.message(lang="uk")

---

MessageReference recognition

MessageReference references on another Message with VariableReferences. This Message must exist in code, or it will be commented and will show error FTLExtractorCantFindReferenceError. In example test-message references on show-lang with VariableReference "$lang".

show-lang =
    { $lang ->
        [uk] { $lang }, Ok
       *[other] { $lang }, 🤔
    }
test-message = Some text { show-lang }

async def another_place(i18n: Any) -> None:
    i18n.show.lang(lang="uk")

async def test(i18n: Any) -> None:
    i18n.test.message(lang="uk")