feat(file): add ReadFileSplit / ReadFileWithReaderSplit (#719)

[ChrisJr404]

Closes #719.

What

Two new helpers in `file/file.go` for streaming values from a list-style file where each line may contain one entry or multiple separator-delimited entries:

```go
// Read each line, then split on every rune in separators, trim whitespace,
// drop empties, stream the result.
ReadFileSplit(filename string, separators ...rune) (chan string, error)
ReadFileWithReaderSplit(r io.Reader, separators ...rune) (chan string, error)
```

When no separators are passed, behaviour reduces to the existing `ReadFile` / `ReadFileWithReader` plus `TrimSpace`. So consumers in httpx / nuclei / similar can call the splitting variant unconditionally and get the strictly-line-based reading for free when they don't pass `','`.

Why

Per @dogancanbakir on httpx#2351:

The resolver file format (one resolver per line) is common to all tools. If we want to support comma separated resolvers on the same line within file, maybe we should move this to utils.

Yes — having it once in `projectdiscovery/utils` lets every PD tool consume the same parsing without copy-pasting the split-and-trim logic each time.

Tests

8 new unit tests in `file/file_split_test.go` covering:

• `splitLineByRunes` with no separators (line-only behaviour)
• comma + multiple separators
• `ReadFileWithReaderSplit` with no separators / with a comma
• `ReadFileSplit` round-trip against a real `t.TempDir()` file mixing single and CSV lines
• missing-file error path
• `ReadFileSplit` with no separator equals `ReadFile` semantics

All new tests pass; existing `file/` package tests (including the `FuzzSafeOpen` fuzzer) still pass:

```
$ go test ./file/...
ok github.com/projectdiscovery/utils/file 20.629s
```

Notes

• Pure-additive: no existing function signature changes; `ReadFile` etc. behave identically.
• The split is rune-based via `strings.FieldsFunc` so callers can pass any of `',', ';', '\\t'` etc.; the typical caller will pass just `','`.
• Whitespace is trimmed and empty pieces are dropped so `"a, ,b"` yields exactly `["a","b"]` without empty noise.