Add reporters config and more documentation

[LucasHillDex]

Summary

This PR adds configurable built-in coverage reporters to the Testem middleware and updates the docs around coverage setup.

What changed

Added a new reporters middleware option so consumers can choose which Istanbul-native reporters to generate.
Kept existing behavior as the default when reporters is not provided.
Preserved compatibility for the text summary file in the default path, and when the text reporter is explicitly requested.
Added validation for the new config so invalid reporter values fail clearly.
Added integration coverage for a custom reporter configuration using json-summary and lcov.
Expanded the README to document:
the new reporters option
how reporter selection works
the requirement to enable build.sourcemap: true in Vite so coverage maps back to source correctly
the need to set distDir when the Vite build output is not dist

Why
This makes coverage output configurable without needing custom post-processing for standard Istanbul report formats, while keeping the current default experience unchanged for existing users.

[NullVoxPopuli]

lgtm, tyty