[main] optparse: add support for prefix flags

[silverweed]

TL;DR: adds support for treating -Dfoo as -D foo. See header documentation for details.

EDIT: superseded by #20947

Checklist:

• tested changes locally
• updated the docs (if necessary)

[devajithvs]

I feel like this approach is a bit too limiting. If you have a short-flag starting with a certain character, you can't define any other option that starts with the same character.

In LLVM, they have a specific argument you can pass when creating a prefix option, something like:

gOptPPDefines("D", cl::Prefix, ...);

Could we support something similar? For example:

opts.AddFlag({"-D"}, ROOT::RCmdLineOpts::EFlagType::kPrefixArg, ...);

[github-actions]

Test Results

    11 files      11 suites   1d 6h 17m 38s ⏱️
 3 786 tests  3 786 ✅ 0 💤 0 ❌
36 183 runs  36 183 ✅ 0 💤 0 ❌

Results for commit 2f2f38e.

[silverweed]

@devajithvs see #20947