feat(manager/gradle): Support properties map in gradle kts

[guicamest]

Changes

Adds support for properties accessor. Gradle ref available here (thx @Churro)

Context

Related to this discussion.
TL;DR;

Given a gradle.properties file that declares a version for a dependency:

...
mockitoKotlin=4.0.0

and a build.gradle.kts that declares a dependency that uses that property via the properties map:

dependencies {
   ...
   testImplementation("org.mockito.kotlin:mockito-kotlin:${properties["mockitoKotlin"]}")
   ...
}

I would like Renovate to raise a PR to update mockitoKotlin to the latest version (current is 4.1.0)

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

Sample repository: https://github.com/guicamest/gradle-kts-properties-renovate-repro

[renovate-release]

🎉 This PR is included in version 35.73.0 🎉

The release is available on:

• GitHub release
• 35.73.0

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