fix: avoid crash when contributors tag is at the start of a file

[davidpavlovschi]

What:

Fixes the crash in all-contributors add/generate when the file being updated starts immediately with the list tag (no title or any content before it — the p5.js CONTRIBUTORS.md case from #376). As reported in the issue:

RangeError: Invalid count value: -1
    at String.repeat (<anonymous>)
    at .../dist/generate/index.js:19:186
    at module.exports (.../dist/generate/index.js:75:78)

Closes #376

Why:

In injectListBetweenTags (src/generate/index.js), nbSpaces is derived from lastIndexOf('\n', startOfOpeningTagIndex). When the tag is at the very start of the file, lastIndexOf returns -1, Math.max(0, …) clamps it to 0, so nbSpaces is 0 and ' '.repeat(nbSpaces - 1) throws RangeError: Invalid count value: -1. For a tag at column 0 on any later line, nbSpaces is 1 and the same expression is repeat(0) — so a zero indent is exactly the established behaviour for an unindented tag; only the file-start position crashes.

How:

Clamp the count: ' '.repeat(Math.max(0, nbSpaces - 1)). No existing output changes — every current snapshot passes untouched. The sibling badge path (replaceBadge, ' '.repeat(nbSpaces)) subtracts nothing and cannot go negative, so it is deliberately left alone.

Added a regression test in src/generate/__tests__/index.js that runs generate() over content whose first byte is <!-- ALL-CONTRIBUTORS-LIST:START --> and asserts a table is injected. It fails on main (reproducing the same RangeError at the String.repeat frame, seen from the test runner rather than the built dist/ bundle) and passes with this change.

Verified locally with npm ci: npx vitest run — 15 files, 114 tests, all passing, no snapshot updates; npx eslint src/generate and npx prettier --check clean.

Checklist:

• Documentation N/A
• Tests
• Ready to be merged
• Added myself to contributors table N/A