Export resolveTemplateUsageCheckString for HMR plugin use

Editor assessment

A correctly composed feat: exporting a new function is an API addition, so `feat(compiler-sfc)` will drive a minor bump and place this in the changelog's features section, which is exactly what downstream HMR plugin authors need to notice. The subject names the exported symbol `resolveTemplateUsageCheckString` verbatim, excellent for grep, and qualifies the consumer ('for HMR plugin use'), giving motivation inline. It is, however, about 78 characters, slightly past the 72 threshold, so it may truncate in compact views; moving 'for HMR plugin use' into a body would fit. No body exists, so the intended integration pattern is not documented, and the PR reference supplies whatever context exists. A strong message with a length nit.