Fix typos in usage message and comments across multiple files

[alexandear]

This PR fixes typos in the filename, comments, and test name.

[Copilot]

Pull request overview

This PR cleans up spelling/typos in user-facing CLI usage text, test naming, and inline comments, improving readability and consistency across the project.

Changes:

• Fixes typos in comments in phonenumbers.go and insertablebuffer.go.
• Corrects a test function name typo in phonenumbers_test.go.
• Fixes a typo in the phoneparser CLI usage message and adds metadata.go embedding declarations.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
phonenumbers.go	Corrects spelling in internal comments.
phonenumbers_test.go	Renames a test to fix a typo in the function name.
metadata.go	Introduces embedded metadata declarations for packaged data files.
insertablebuffer.go	Corrects spelling in a comment (noted remaining informal phrasing).
cmd/phoneparser/main.go	Fixes a typo in the CLI usage string.

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