rebar3 plugin entry point for erli18n catalog tooling.

init/1 is the single hook rebar3 calls when it loads the plugin. It chains the four providers' own init/1 functions, each of which registers its provider under the erli18n namespace, so that after load the project has:

• rebar3 erli18n extract — walk abstract forms into .pot templates;
• rebar3 erli18n merge — msgmerge-style .po sync;
• rebar3 erli18n check — CI gate that fails on .pot drift;
• rebar3 erli18n report — per-(Domain, Locale) completeness.

The plugin is a SEPARATE Hex package from the runtime erli18n library and depends on it ({deps,[{erli18n,"~> 0.5"}]}, {applications,[...,erli18n]}), pointing plugin -> lib — the same direction as rebar3_gpb_plugin -> gpb. The providers reuse the published PO read/serialize API (erli18n_po:parse/1, erli18n_po:dump/1, erli18n_po:escape_string/1) across that boundary. Consumers opt in with {plugins,[rebar3_erli18n]} in their own rebar.config. Compile-time .po->BEAM codegen is out of scope for this plugin: lookups resolve catalogs at runtime, so no codegen provider is shipped.