Fix -comment flag handling for multi-line values

[stevendpclark]

Summary

• When the -comment argument contains newline characters, the generated file now emits one // -prefixed line per segment instead of embedding the raw newline inside a single comment line (which produces invalid Go)
• The same newline-in-args edge case is sanitized in the DO NOT EDIT header line by collapsing embedded newlines to spaces
• Replaces deprecated ioutil.TempFile with os.CreateTemp (deprecated since Go 1.16)

Root cause

g.Printf("// %s\n", comments.String()) emitted the comment as a single Printf call. If the string contained \n, the output would be:

// line one
line two // ← not a comment — syntax error

Fix

Split the comment string on \n and emit each segment as its own // … line.

Test plan

• TestCommentWithNewline — end-to-end test: builds the enumer binary, generates a file with -comment "line one\nline two", asserts both comment lines appear in the output, and verifies the generated file compiles cleanly