Make - and _ synonyms in project options and enforce a convention for option names

Since we didn't specify a convention, people have been inconsistently using `-` or `_` in different projects for options. This is particularly annoying in umbrella projects like GNOME where support for the same library is under different options in different projects.

I also think we should decide on a convention for meson options, provide a way to migrate options (via an alias that either warns or errors), and enforce it. For instance, options must not start with enable/disable or with/without since those are redundant. You need those in Autotools because it does not have 'types' for option values, but we do.

We should do this sooner rather than later.

The issue names no files, tests, or entry points. Start by surveying Meson’s project-option parsing and validation; done would require an agreed naming convention, a migration alias that can warn or error, and enforcement against redundant enable/disable or with/without prefixes.

Written by the indexing model from the issue text.