feat(gradle): gradle plugin dependency versions in variables (#6063)

[Jnosh]

Adds support for gradle plugin dependency versions specified as variables.
Plugin versions can be set using variables inside a pluginManagement block in settings.gradle[.kts].

Example (kotlin script):

pluginManagement {
    plugins {
        val myPluginVersion = "1.0.0"
        id("my.plugin.group") version myPluginVersion 
    }
}

I've essentially just added new matchers based on the existing support for plugin versions specified as strings and the version variable support for non-plugin dependencies.

Notes:

• I've had to adjust the regex created by moduleStringVersionFormatMatch to not allow " in version names to prevent it from matching """$versionVar""" constructs.
• The regexes for matching the version variables require whitespace, a ; or a } after the variable name since the variable name can occur without any other clear end marker. Since these expressions should occur inside of a plugins block, there should always be whitespace, a line terminator or the end brace of the plugin block after a valid definition.

Closes #6063

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[renovate-release]

🎉 This PR is included in version 19.223.0 🎉

The release is available on:

• GitHub release
• 19.223.0

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