Fix IMAP keyword flag handling

[mpscholten]

Summary

Fix keyword flag handling according to RFC 3501 section 2.3.2:

• show (Keyword "Custom") now renders Custom instead of \Custom
• bare keyword flags still parse as keywords
• unknown backslash-prefixed system flags are preserved distinctly instead of being collapsed into bare keywords

Fixes #1.
Fixes #2.

Checks

• nix shell nixpkgs#cabal-install nixpkgs#ghc --command cabal test imap-parsers --test-show-details=direct
• nix shell nixpkgs#cabal-install nixpkgs#ghc --command cabal build all
• nix shell nixpkgs#cabal-install nixpkgs#ghc --command cabal check (existing CHANGELOG placement warning only)
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the \ wildcard when showing flags*

When a server advertises PERMANENTFLAGS (\\*), this parser still represents the wildcard as Keyword "*" and uses that sentinel to set _isFlagWritable, but the new catch-all Show case now renders that same value as bare * instead of \\*. Bare * is not a valid keyword here (atomChar excludes *), so any caller that logs, compares round-trips, or serializes this exposed Flag value now gets an invalid IMAP flag string; special-case Keyword "*" before the bare-keyword case.

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