Made mutex member variable mutable

Classes inheriting from `base_sink` can now lock the base mutex inside their `const` member methods (e.g., basic accessors).

fix Xcode compiler warning "no new line at the end of file"

fix bug #1790

Minor update, PEP8 format and f-string instead of format method

Update pattern_formatter-inl.h full_formatter comment to describe the default logging pattern expression accurately

Comment describing the default logging pattern is missing [%s:%#], which is the abbreviated source file + the line number.

I tried to customize our own logger by copy pasting this info, and then I noticed we had lost that information.

Minor typo

Update .clang-tidy

remove constexpr on level_string_views to fix compilation on C++17 fr…

Remove version requirement from fmt find_package

By introducing 'no module' support this inadvertently triggered the find_package to use the "full version" version selection.

From https://cmake.org/cmake/help/latest/command/find_package.html#version-selection:

> When the [version] argument is given, Config mode will only find a version of the package that claims compatibility with the requested version (see format specification).

FMT does not set this configuration https://github.com/fmtlib/fmt/blob/7.1.3/support/cmake/fmt-config.cmake.in... but regardless it would not be "any compatibility" based on version semantics.

Which causes this error. v1.8.2 builds perfectly fine.

```
CMake Error at CMakeLists.txt:181 (find_package):
  Could not find a configuration file for package "fmt" that is compatible
  with requested version "5.3.0".

  The following configuration files were considered but not accepted:

    /home/proj/build/e98598522b7c484d9220bffc7ec84474515facbc/fmt-config.cmake, version: 7.1.3
```

Add required libraries for Android CMake build

added spdlog::level::set_string_view to enable alternate log level na…

added spdlog::level::set_string_view to enable alternate log level names without changing the build via SPDLOG_LEVEL_NAMES

add default cases

Removing changes to the bundled fmt library. The default case
statements will be changed in the upstream library.
Signed-off-by: Ryan Sherlock <sherlock@loftorbital.com>

Adding default case for cases where the compilation flag
-Wswitch-default is present on the command line when spdlog is
included in external projects.
Signed-off-by: Ryan Sherlock <ryan.m.sherlock@gmail.com>