fix: improve log line level detection

[trevorwhitney]

What this PR does / why we need it:

I've dealt with a few support requests / bug reports about log line level detection. This PR attempts to improve our detection code a bit to capture a few more cases. The logic changes slightly to,

• does the level keyword exist on it's own, and not part of another word?
  • if so, we use that level
  • if not, we don't use that level

we're still using standard string search / indexing, and will return the first level that matches, so performance should be about the same. I will attach benchmarks to the PR once I have a chance to run time.

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR