Introduce solidity.evmVersion() to query the target EVM

Introduce `solidity.evmVersion()` function which returns an enum of `enum solidity.EvmVersion { Homstead, TangerineWhistle, SpuriousDragon, ... }`.

This would mean the `solidity` identifier is reserved for "language magic", but we could consider such functions to be explicitly importable instead from the special solidity source:
```
import evmVersion from solidity; // notice that `solidity` is not a literal, but an identifier
```

This would be useful for implementing features in #10282.

Start by reviewing how the special `solidity` source and language-magic identifiers are handled. Compare the proposed `EvmVersion` enum with the compiler's target EVM version support, and resolve whether the API or explicit import form is intended. Done means the selected interface can report the target EVM version and supports the feature described in #10282.

Written by the indexing model from the issue text.