New process for experimental features

Implement changes needed to introduce the [New process for experimental features in Solidity](https://notes.argot.org/s/@solidity-experimental-features-new-process) that we have decided to adopt.

- [x] Introduce `--experimental`/`settings.experimental` in the compiler
- ~Make `--help` output dependent on the flag~
- [x] Include experimental features in `--help` and Standard JSON input docs and clearly mark those that are already there as experimental.
- [x] Gate all the experimental features behind the flag:
- AST import
- EVM Assembly Import
- IR AST output
- Experimental EVM versions
- Ethdebug
- SSA CFG
- (optional) Generic Solidity prototype
- (optional) LSP
- [ ] Add the list of experimental features to metadata.
- [x] Introduce `experimental` EVM version for testing purposes. https://github.com/argotorg/solidity/pull/16443
- [x] Fix #15835
- [x] Create a documentation page about experimental features:
- Our process around them, including lack of any stability guarantees.
- How to enable experimental features.
- List of all features that are currently considered experimental.
- [x] Add a pre-release section to the release checklist.
- [x] Make sure our scripts can handle the `v-pre.` scheme for pre-release versions:
- `get_version.sh`
- scripts in solc-bin
- scripts in solc-js

Use the unchecked checklist item—adding the list of experimental features to metadata—as the remaining scope. First locate where compiler metadata is defined and generated, then compare the existing experimental-feature list with the metadata output; done means the list is present and accurately reflects the features described in this issue.

Written by the indexing model from the issue text.