core: file: add writing method

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
core: add mode to files

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
tests: test new file features

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
tests: improve file tests

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
core: file: add skip and delete

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>