feat(gradle): add support for simple extra properties

[Churro]

Changes

• Adds parsing support for simple extra properties that are declared using the ext prefix.

Minimal reproduction repository:

• https://github.com/Churro/renovate-gradle-ext/blob/master/build.gradle
• https://github.com/Churro/renovate-gradle-ext/pull/1

Context

• Related to in Resolve dynamic deps versions in gradle #15359 but not solving the problem described there.
  • Parsing library versions in ext hierarchies like the one exemplified in that issue will probably only work once https://github.com/zharinov/good-enough-parser is integrated

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 tick 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

[viceice]

🤷‍♂️ code LGTM

[renovate-release]

🎉 This PR is included in version 32.81.0 🎉

The release is available on:

• GitHub release
• 32.81.0

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