std.conv: add writeText, writeWText, writeDText

[pbackus]

These are variants of text, wtext, and dtext that write their output to an output range instead of returning a string.

Fixes #10550

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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 + phobos#10652"

[pbackus]

Style check is going to fail because of dlang-community/D-Scanner#956

[pbackus]

CC @LightBender

[LightBender]

Naming nit:
writeWtext = writeWText
writeDtext = writeDText

[pbackus]

The current names were chosen to be consistent with text, wtext, and dtext, which do not capitalize the word "text." I don't mind changing them if we think quality of new names is more important than consistency with old ones, though.

[LightBender]

In v3 those methods are getting renamed. Likely to something like asText, asWText, and asDText. There is a thread about these names in the v3 design repo.

[pbackus]

Ok; I'll assume you're bringing this up because you think that we should also follow this Phobos v3 naming convention when adding new symbols to Phobos v2, even if it breaks consistency with existing v2 symbols.

[LightBender]

TBH, I've gone round and round on the idea of V2 names following the V3 conventions even if that breaks consistency, both with others and with myself.

And where I've landed on it is: Every name that does not change between versions is one less bit of friction to the process of upgrading your code to V3. It's already going to be an unpleasant chore, anything that makes it easier will help convince people to make the switch.