[#4] Fix found character that cannot start any token

Dylan Wilson requested to merge issue-4 into main

Associated issue: #4 (closed).

Since YAML 1.1, a percent sign (%) cannot start any unquoted string as %YAML can be used to denote the version of YAML in a given document and %TAG is used as a shortcut for URI prefixes.

Quote all uses of percent sign in double quotes.

Fixes #4 (closed).

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