Omitting the visibility modifier creates a Groovy _property_ [1], which
is composed by:
* a _private_ backing field
* a public getter
* (+ setter if the property is not `final`)

It is unlikely to be an intended design as it makes access to those
fields awkward from statically typed languages
`ShadowBasePlugin.getCONFIGURATION_NAME()`.

This commit replaces such _properties_ with _public fields_ only.

This is a backward incompatible change as this removes public getters.

Here is what the generated class looked liked before this
commit (7.0.0):

```
$ javap -p -c -constants ShadowBasePlugin.class
Compiled from "ShadowBasePlugin.groovy"
public class com.github.jengelman.gradle.plugins.shadow.ShadowBasePlugin implements org.gradle.api.Plugin<org.gradle.api.Project>, groovy.lang.GroovyObject {
  private static final java.lang.String EXTENSION_NAME = "shadow";

  private static final java.lang.String CONFIGURATION_NAME = "shadow";
[...]
  public static java.lang.String getEXTENSION_NAME();
    Code:
       0: getstatic     GradleUp#86                 // Field EXTENSION_NAME:Ljava/lang/String;
       3: areturn

  public static java.lang.String getCONFIGURATION_NAME();
    Code:
       0: getstatic     GradleUp#111                // Field CONFIGURATION_NAME:Ljava/lang/String;
       3: areturn
```

[1] https://groovy-lang.org/objectorientation.html#properties