File level lint warnings are not reported

[pkwarren]

Currently, file-level lint warnings are not reported by the plugin. To fix, we should make the following changes:

• BufIssue.toTextRange returns null if the start/end offsets are equal, which file-level lint warnings are (they're reported with start/end line and start/end column all set to 1). We should detect thse any report annotations as fileLevel() instead of range(...).
• The IgnoreBufIssueQuickFix can't be used to ignore these lint failures for these types of errors. We should instead update the buf.yaml config file with https://buf.build/docs/configuration/v1/buf-yaml#ignore_only or come up with another appropriate fix.

This affects the following types of lint warnings:

• PACKAGE_DEFINED
• FILE_LOWER_SNAKE_CASE