Introduce pragma experimental solidity

[nikola-matic]

Note: this is still a work in progress

• Changelog entry (pragma, ast)
• Decide feature name for the experimental pragma (current placeholder is next) (decision pragma experimental solidity)
• Change pragma from next to solidity
• Resolve imports in CompilerStack

Fixes: #10639 (comment)

[ekpyron]

I'd change the changelog phrasing and would still change the flag name and not always emit it in the json AST, but only when it's true.
Apart from that mostly optional comments and almost good to go.
Testing is good enough (if you want, you can still add tests for it occurring twice in the beginning and once in the beginning plus once later on - but since that's all covered by the preexisting logic, it's also fine if not - the same goes for the experimental flag in CBOR metadata: could be tested specifically, but it's already tested for any experimental pragma, so also fine)

[r0qs]

Nice! Just some questions :)

[nikola-matic]

Testing is good enough (if you want, you can still add tests for it occurring twice in the beginning and once in the beginning plus once later on - but since that's all covered by the preexisting logic, it's also fine if not - the same goes for the experimental flag in CBOR metadata: could be tested specifically, but it's already tested for any experimental pragma, so also fine)

@ekpyron precisely why I didn't add such tests, as they would be completely redundant, i.e. adding them for experimental solidity covers the same path as experimental ABIEncoderV2, or SMTChecker.

[ekpyron]

Only minor adjustments - if you fix the comments above and squash, we can merge today.

Also needs a rebase, though, and the Changelog entries needs to move to 0.8.21.

Premature approval.

[ekpyron]

Minor cosmetics left, if you please, but generally good to merge.