Support a user/global repository mirror configuration

[chengjilai]

Context

Repositories are currently configured per-project only, in module.yaml repositories:. To use a mirror (e.g. for mainland China, where maven.google.com and repo1.maven.org are unreachable) every project must repeat the block:

repositories:
- id: mavenGoogle
url: https://cache-redirector.jetbrains.com/maven.google.com
- id: mavenCentral
url: https://cache-redirector.jetbrains.com/kotlin/repo1.maven.org/maven2

There is no user-level or global default for repositories (no env var, no config file under ~/.config or the toolchain cache dir - verified in the source). Mirror settings therefore drift across projects and must be re-declared on every machine.

What is needed

A user/global default repositories configuration, e.g.:

• a config file (e.g. ~/.config/kotlin-toolchain/repositories.yaml or similar), and/or
• an environment variable listing default repository URLs.

Project-level module.yaml repositories should still override or extend the global defaults (the existing dedup-by-id behavior makes this easy: global defaults first, project entries win).

Expected behavior

Set mirrors once per machine (or in CI env), and every kotlin project resolves through them without repeating the repositories block.