fix: Remove `is_text_char_restricted`.

[duesee]

Note: We need to resolve the Code ambiguity differently. Closes #299.

~This will lead to false-positives during fuzzing. However, usability trumps "correctness" here and we can revisit this after we confirmed that everything else works with Gmail.~ Fixed Arbitrary implementation for now.

[coveralls]

Pull Request Test Coverage Report for Build 5456069513

• 51 of 52 (98.08%) changed or added relevant lines in 4 files are covered.
• 6 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.05%) to 93.242%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imap-types/src/arbitrary.rs	29	30	96.67%
<!--	Total:	51	52	98.08%	-->

Files with Coverage Reduction	New Missed Lines	%
imap-codec/src/response.rs	1	98.9%
imap-types/src/extensions/quota.rs	1	90.7%
imap-types/src/body.rs	2	39.47%
imap-types/src/flag.rs	2	62.5%
<!--	Total:	6		-->

Totals
Change from base Build 5445090950:	-0.05%
Covered Lines:	9092
Relevant Lines:	9751

---

💛 - Coveralls

[duesee]

Enter IMAP4REV1 response (or "exit"): * OK [HIGHESTMODSEQ 13637]
Status(
    Ok {
        tag: None,
        code: None,
        text: Text(
            "[HIGHESTMODSEQ 13637]",
        ),
    },
)

Okay, IMAP ambiguity + non-standard response in Gmail is... something. Use cut as soon as we are parsing a Code?