Make safe error messages consistent

[dkorpel]

There are quite a few safety error messages, and some tell they are related to 'safe code' or @safe functions, while others don't. This is not only inconsistent, it's also false with -preview=safer: some of these errors occur in functions without a @safe annotation.

This PR rewrites all safety error messages so they either form a noun or gerund: "pointer arithmetic" or "void initializing a pointer". The error reporter will then surround that in context:

deprecations: "{action} will become @system in the future"
safe functions: "{action} is not allowed in a @safe function"
safer functions: "{action} is not allowed in a function with default safety with -preview=safer"
system variables: "{action} can't initialize @safe variable %s"
inferred supplemental error: "which wasn't safe because of: {action}"

Occasionally this requirement results in suggested fixes being phrased slightly awkwardly:

Error: using `b.ptr` (instead of `&b[0])` is not allowed in a `@safe` function

A future enhancement of adding supplemental errors to setUnsafe could improve this.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#20654"

[ibuclaw]

Hi @dkorpel - this caused a diagnostic regression - there's a missing closing backtick in this format string.

Are you able to fix this? Alternatively can include it in the bundle of "broken DMD/C++ API" things I'm preparing.

[dkorpel]

Yes, give me a minute

[dkorpel]

#20677