feat: add zls toolchain support

[cerisier]

Closes #403

First WIP of porting zls support to rules_zig.

I followed our discussion of declaring dedicated ZLS toolchains

• One ZLS toolchain maps to a version of Zig which the user can choose.
• It is on purpose not done automatically since users might want to override those with HEAD versions.

Those are configured in a dedicated zls extension with similarity with zig extension in how we specify version, index and how it generates repos.

This is WIP.