fix some issues with --backend

[timotheecour]

before this PR, commands like nim doc -b:cpp didn't work when the backend (eg cpp) required adjustments (such as conf.exc = excCpp), as was noticed in #14349)

this PR fixes that and other issues with --backend:

nim $cmd --backend:$backend now works reliably by splitting the backend handling logic into customizeForBackend (called for all commands) vs compileToBackend (only called for commands that compile to a backend);

• fix nim c -b:cpp main doesn't work timotheecour/Nim#175: --backend can safely override previous options so that for eg nim c -b:cpp main and nim c -b:js -b:cpp main end with with cpp backend selected, and so that defined(c) and defined(js) would be false (no-side-effect rule)

parsing the flag --backend has by design zero side effect beyond setting conf.backend, so that it can be overridden in subsequent flags and plays well with other commands

• update changelog (should've been done in nim doc --backend:js, nim doc --doccmd:-d:foo, nim r --backend:js, --doccmd:skip + other improvements #14278, made it up to date)

see tests

CI failure unrelated (#13166) re-ran