fix: allow numeric output filenames

[puneetdixit200]

Fixes #62.

Summary

• Allow Output paths that start with numeric lexer fragments, such as 1.gif.
• Recombine adjacent numeric/string path tokens before determining the output extension.
• Add parser regression coverage for numeric output filenames.

Tests

• Red before fix: go test ./parser -run TestParseOutputNumericFilename -count=1 failed with Expected file path after output and invalid 1. / gif commands
• go test ./parser -run TestParseOutputNumericFilename -count=1
• go test ./parser -count=1
• go test ./...
• git diff --check