docs: document --parents flag for files.write command

Also adds a test.

Already supported in `js-ipfs`, `go-ipfs` requires ipfs/kubo#5359
to be merged and released

SPEC/FILES.md

@@ -875,6 +875,7 @@ Where:
   - `offset` is an Integer with the byte offset to begin writing at (default: 0)
   - `create` is a Boolean to indicate to create the file if it doesn't exist (default: false)
   - `truncate` is a Boolean to indicate if the file should be truncated after writing all the bytes from `content` (default: false)
+  - `parents` is a Boolean value to decide whether or not to make the parent directories if they don't exist (default: false)
   - `length` is an Integer with the maximum number of bytes to read (default: Read all bytes from `content`)
   - `raw-leaves`: if true, DAG leaves will contain raw file data and not be wrapped in a protobuf (boolean, default false)
   - `cid-version`: the CID version to use when storing the data (storage keys are based on the CID, including it's version) (integer, default 0)

js/src/files/write.js

@@ -48,5 +48,14 @@ module.exports = (createCommon, options) => {
         done()
       })
     })
+
+    it('should write to deeply nested non existent file with create and parents flags, expect no error', function (done) {
+      const testDir = `/foo/bar/baz/test-${hat()}`
+
+      ipfs.files.write(testDir, Buffer.from('Hello, world!'), {create: true, parents: true}, (err) => {
+        expect(err).to.not.exist()
+        done()
+      })
+    })
   })
 }