FeatureCompatibilityVersion::toString and FeatureCompatibilityVersion::parse should be moved to this new parser file.

Along with:

• kParameterName, kVersionField and kTargetVersionField from FeatureCompatibilityVersion.
• kVersion36, kVersion40, kVersionDowngradingTo36, kVersionUpgradingTo40 and kVersionUnset from FeatureCompatibilityVersionCommandParser.

FeatureCompatibilityVersion:: kCommandName should move into feature_compatiiblity_version_command_parser.h/cpp where it logically belongs.

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Reason for rearrangement: to break up dependencies.
feature_compatibility_version.cpp is a pretty high level file, and users of the constants and toString and parse functions can be lower level, sometime necessitating ugly file linking or making it impossible to link with. See SERVER-31968.