astgen: forbid trailing whitespace in multiline strings

[matklad]

Trailing whitespace in multiline strings is annoying for all the usual
reasons trailing whitespace is annoying (unrelated line changes).

However, in this case it even can leak into whatever the string literal
is used for. For example, in the previous commit there is trailing
whitespace leaking into the help messages.

So it makes sense to forbid this outright.

zig fmt intentionally doesn't try to clean up here --- if there's a
trailing whitespace in a multiline literal, it is ambiguous whether
the user added that by accident, or whether they intended to have a
space there.

If you need to have trailing whitespace, you can contatenate \\ and
" strings with ++:

const S =
    \\hello
++ "  \n" ++
    \\world
    \\
;

Closes: #19299

[matklad]

Note: that proposal is neither accepted not rejected. This PR shouldn't be considered as an argument in favor of accepting it. Rather, if we do accept it, than that's how an implementation could look like.

That being said, I am strongly in favor of accepting this, as I've hit the problem of trailing whitespace leakage a couple of times.

[andrewrk]

OK, I'll make an effort to reach a decision on that proposal since you already have a working PR 👍