Simplify create and use more robust defaults.

[joshuaauerbachwatson]

This PR addresses issue #185 as it has evolved to encompass several changes. The changes affect nim project create, nim plugins, and nim project update.

1. A created project always places the sample code in a subdirectory named for the action rather than putting a source file directly in the package directory. This avoids some common errors as the developer changes the sample into something real.
2. The default package is no longer used because it is not a "real" package and some options won't work with it. Instead, the package is called sample. This avoids some other speed-bumps that tend to arise later in development.
3. We add a minimal .gitignore to the created project. It at least excludes the most common artifacts that you wouldn't want to commit.
4. A project.yml is always generated. The --config flag is still accepted but is hidden and has no actual effect.
5. The postman and openapi plugins are temporarily retired, to be re-introduced at some future date under a new nim project import command. As a consequence:
   • The --type flag on create is no longer recognized.
   • The nim project update command is removed. In the absence of the plugins it is essentially useless due to bugs and restrictions. It has historically been hidden in any case. The --overwrite flag on nim project create is retained but, as always intended, it has no "merging" behavior (does a complete replacement).
   • The plugins plugin and the nim plugins commands are removed.

As part of this change, plugin-autocomplete was also removed. It continued to be very hard to support due to bash versioning issues.

[joshuaauerbachwatson]

As noted in issue #185, support for C# as a sample language has been withdrawn for the moment. It appears likely that it never worked quite right, as revealed by the new tests that were added by this PR.