Support for project-local migration templates

When running migrations on production databases with active traffic, it's important to include safety settings like SET lock_timeout = '5s'; to prevent migrations from blocking writes indefinitely while waiting for locks.

Currently, teams must remember to manually add this to every migration - which is error-prone. A simple template feature would allow enforcing safe defaults automatically.

Proposed solution:

If a template file exists in the project (e.g., db/.dbmate/template.sql or configured via --template-file), dbmate new would use it instead of the default empty template:

-- migrate:up
SET lock_timeout = '5s';

-- migrate:down

Why this differs from PR #530:

The closed PR stored templates in ~/.config, which goes against dbmate's KISS philosophy. This proposal keeps templates in the project repo, making them:

• Version controlled with the project
• Shared across the team automatically
• Consistent with dbmate's simple, portable approach

This is a small change that would help teams enforce safe migration practices without external tooling.