Move `PGBuild.target` and `PGBuild.optimize` to `PGBuild.options`
struct.

Introduce `PGBuild.modules` struct to load a the exported modules via
simple function calls like `pgbuild.modules.pgzx()`.

Rename `Run` to `RunExec` to better indicate what that step does.

Simplify addRegress by removing the RunRegress type.

Introduce `addExtensionLib` to the shared lib to compile without
installing it right away. This allows users to modify/use the lib with
other targets that are not required to install the extension. For
example a check target or build source documentation without linking and
installation.

Introduce `Project` to combine the meta-data into a common type that can
be used to create the different project resources within the build
script. E.g. In order to create build/check/docs only steps one is
required to redeclare the extension library to have an independent
resource. Using the `Project` type we can use `proj.extensionLib` to
create multiple instance of the build configuration (including
dependencies).

Update the example build scripts:
- improve structure by pre-declaring build steps
- Use new `Project` type.
- Add `zig build check` command that compiles and type checks, but
  ommits LLVM and linking. Use with ZLS to improve developer experience
  like: https://kristoff.it/blog/improving-your-zls-experience/