Add inherit: false to material2018 text geometry text themes

[LongCatIsLooong]

Fixes #103864

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[HansMuller]

I don't recall why the 2018 text styles didn't specify inherit: false in the first place. Perhaps we'll never know. Or maybe it was just an oversight.

[rydmike]

Nice this is awesome and a surprisingly simple fix. I had no idea it was allowed to change the framework to set it to false. 😃

Thanks @LongCatIsLooong, many others have reported this issue too and it has been open a long time. We should close the duplicates/related issues too, if not done already (automatically?). I think most of them are referenced in comments to my issue report.

I made a slightly tedious workaround in Themes Playground to get around this issue. I needed a workaround since the app needs to be able to dynamically switch between 2018 and 2021 Typography, or at least make it looks like it does so. So I made it stay in 2021 all the time, and simulate switching, by instead changing to an internal TextTheme in 2021 Typography, that looks like it is using 2018 Typography. The app's code gen output still gives a setup for the actual 2018 Typography, when that is selected. While it worked, and users don't see and know it is going on, I will be happy to remove its extra code once this lands in stable.

Love your honest comment @HansMuller, I would not expect either that it would be possible to know and remember historic rationale for everything in the framework, it is pretty darn big. Sure oversights might happen too... naah not really in Flutter framework, the people working on it are just too awesome 😃💙