[Syntax]: Adopt gamecoe's naming convention

[nircoe]

logcoe used camelCase/PascalCase/s_-prefixed naming and a LoggerImpl class for its internal state.
This renames everything to gamecoe's snake_case/g_-prefix convention and replaces LoggerImpl with free functions over anonymous-namespace globals.

• Prefix cmake/utils.cmake's functions with logcoe_
• Rename the public API to snake_case (LogLevel to log_level, function and parameter names)
• Replace LoggerImpl with free functions over anonymous-namespace globals, wrapped in internal::
• Update tests to the renamed API
• Update README.md, CONTRIBUTING.md, and ARCHITECTURE.md for the renamed API and structure, and document the snake_case/g_-prefix convention
• Fix data races in flush() and the default log source lookup by holding the mutex for their full duration
• Fix an integer underflow in shutdown() when called without a prior initialize()