Multi-version configuration files

Currently, the config file in $XDG_CONFIG_HOME/dune/config sets a lower bound on the Dune version useable on one's machine (via the lang stanza):

File "/home/.config/dune/config", line 1, characters 11-14:
1 | (lang dune 2.1)
               ^^^
Error: Version 2.1 of dune is not supported.
Supported versions:
- 1.0 to 1.12
- 2.0

In order to access the latest features (such as the cache), I need to update all projects on my machine in lockstep. I think it would be nice to allow projects to be upgraded independently while allowing access to the newest features. Here's two potential strategies:

Proposal 1 - Multi-version config file

We could make all configuration settings forwards-compatible by adding some sort of conditional stanza:

(for-version 
 (>= 2.1.0)
 (cache enabled))

• advantages: retains a single source of truth for Dune config.
• disadvantages: makes it harder to change the config file parser in future, and makes the lang stanza in the config files mostly redundant.

Proposal 2 - Multiple config files

We could have version-specific config files that are read in some sort of preference order, for instance:

$XDG_CONFIG_HOME/dune/config.2.0.1  (* Read by >= 2.0.1 *)
$XDG_CONFIG_HOME/dune/config.1.11.2 (* Read by >= 1.11.2 & < 2.01 *)
$XDG_CONFIG_HOME/dune/config        (* Read by < 1.11.2 *)

• advantages: logic for parsing the config is the same as before, and retains the same 'lang header' / 'body' parser distinction as the other Dune files.
• disadvantages: config parameters will likely be duplicated in the files.

Personally, I prefer the second option for not violating my expectations of the lang stanza. I'm happy to implement whatever solution is deemed best.

Start by reviewing the configuration file at $XDG_CONFIG_HOME/dune/config and how its lang stanza limits supported Dune versions. Compare the proposed conditional-stanza and version-specific-file approaches, then define how independent project upgrades and access to newer features should work. Done means a decided configuration design that supports multiple Dune versions without requiring all projects to upgrade together.