Fix edge case in path.relative.

[Hanaasagi]

Close: #3924

What does this PR do?

• path.relative("/webpack-hot-middleware", "/webpack/buildin/module.js") should return ../webpack/buildin/module.js

Some explanations.

Assuming the input is as follows:

• input[0] => /webpack-hot-middleware
• input[1] => /webpack/buildin/module.js

bun/src/resolver/resolve_path.zig

Lines 198 to 207 in 928816b

	2 => {
	while (index < min_length) : (index += 1) {
	if (input[0][index] != input[1][index]) {
	break;
	}
	if (@call(.always_inline, isPathSeparator, .{input[0][index]})) {
	last_common_separator = index;
	}
	}
	},

The longest common substring is /webpack, So the index is 8 (point to - or /).

But here

bun/src/resolver/resolve_path.zig

Lines 297 to 303 in 928816b

	for (input) |str| {
	if (str.len > index) {
	if (@call(.always_inline, isPathSeparator, .{str[index]})) {
	return str[0 .. index + 1];
	}
	}
	}

we will get /webpack/ here, a wrong directory.

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

I wrote automated tests

[Jarred-Sumner]

Thank you! Let's see the tests run if anything interesting happens. Changes to path resolution often have a lot of unexpected side effects, though nowadays we have much better test coverage so I expect any issues to come up in tests