refactor: rename apps to extensions

[julienrbrt]

Closes: #4235

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[salmad3]

I believe test is failing because error message still uses "app"

[Pantani]

LGMT

The only thing I saw in the code is that the integration test folder is still integration/plugin. We can move to integration/extension

[julienrbrt]

True, I'll fix!

[Pantani]

ignite/config/plugins/testdata/other/extensions.yml content is also wrong

extensions:
  - name: plugin1
    path: /path/to/plugin1
  - name: plugin2
    path: /path/to/plugin2
    with:
      foo: bar
      bar: baz

[Pantani]

https://github.com/ignite/cli/pull/4238/files#diff-889e5321b1b3424218f8cdaa0c2bb4309c472bd4ab4e91c04554159184c77c9aL301

https://github.com/ignite/cli/pull/4238/files#diff-889e5321b1b3424218f8cdaa0c2bb4309c472bd4ab4e91c04554159184c77c9aL178

"can't attach app hook %q to non executable command %q" error

[Pantani]

still have some merge conflicts

[julienrbrt]

We have discussed internally that we'll not proceed yet with this change. The confusion between Ignite Apps and AppChain will be solved with better usage of the word App.