Add support for meson.options as a replacement for meson_options.txt

We will still try to load `meson_options.txt` if `meson.options` doesn't
exist. Because there are some advantages to using `meson.options` even
with older versions of meson (such as better text editor handling)
we will not warn about the existence of a `meson.options` file if a
`meson_options.txt` file or symlink also exists.

The name `meson.options` was picked instead of alternative proposals,
such as `meson_options.build` for a couple of reasons:

  1. meson.options is shorter
  2. While the syntax is the same, only the `option()` function may be
     called in meson.options, while, it may not be called in meson.build
  3. While the two files share a syntax and elementary types (strings,
     arrays, etc), they have different purposes: `meson.build` declares
     build targets, `meson.options` declares options. This is similar to
     the difference between C's `.c` and `.h` extensions.

As an implementation detail `Interpreter.option_file` has been removed,
as it is used exactly once, in the `project()` call to read the options,
and we can just calculate it there and not store it.

Fixes: #11176

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