This is required to make the various keys in the [user options] section
work the same as they do in the meson_options.txt file, where we don't
have any rules about case sensitivity.

There is some risk here. Someone may be relying on this lower by default
behavior, and this could break their machine files.

Fixes #7731

This attempts to clarify the usage of strings and arrays, as well as
document the boolean type that has been exposed via the project and
built-in options

This is like the project options, but for meson builtin options.

The only real differences here have to do with the differences between
meson builtin options and project options. Some meson options can be set
on a per-machine basis (build.pkg_config_path vs pkg_config_path) others
can be set on a per-subproject basis, but should inherit the parent
setting.

This allows adding a `[project options]` section to a cross or native file
that contains the options defined for a project in it's meson_option.txt
file.

Machine files already supports `+` operator as an implementation detail,
since it's using eval(). Now make it an officially supported feature and
add a way to define constants that are used while evaluating an entry
value.

Rather than having two separate sections with duplicated information
lets just have one for the common settings, and only document sections
specific to each file in separately