Unfortunately, {fmt} allows passing too many arguments to a format call
without raising any runtime or compile-time error [1].

As this is a common source of bugs since we started migrating to {fmt},
this commit adds some custom logic to validate the number of
replacement fields in format strings in addition to {fmt}'s own checks.

[1] https://github.com/fmtlib/fmt/issues/492

Common shouldn't be depending on APIs in Core (in this, case depending
on the PowerPC namespace). Because of the poor separation here, this
moves OSThread functionality into core, so that it resolves the implicit
dependency on core.

I had to rename Source/Common/Compiler.h because the VS build
confuses it with Externals/zstd/lib/common/compiler.h otherwise.

DebugInterface: Add GetThreads

WatchWidget: Update widget on AddWatch