Generate documentation stubs from Javadocs

illuaminate does not handle Java files, for obvious reasons. In order to
get around that, we have a series of stub files within /doc/stub which
mirrored the Java ones. While this works, it has a few problems:

 - The link to source code does not work - it just links to the stub
   file.
 - There's no guarantee that documentation remains consistent with the
   Java code. This change found several methods which were incorrectly
   documented beforehand.

We now replace this with a custom Java doclet[1], which extracts doc
comments from @LuaFunction annotated methods and generates stub-files
from them. These also contain a @source annotation, which allows us to
correctly link them back to the original Java code.

There's some issues with this which have yet to be fixed. However, I
don't think any of them are major blockers right now:

 - The custom doclet relies on Java 9 - I think it's /technically/
   possible to do this on Java 8, but the API is significantly uglier.
   This means that we need to run javadoc on a separate JVM.

   This is possible, and it works locally and on CI, but is definitely
   not a nice approach.

 - illuaminate now requires the doc stubs to be generated in order for
   the linter to pass, which does make running the linter locally much
   harder (especially given the above bullet point).

   We could notionally include the generated stubs (or at least a cut
   down version of them) in the repo, but I'm not 100% sure about that.

[1]: https://docs.oracle.com/javase/9/docs/api/jdk/javadoc/doclet/package-summary.html

doc/stub/fs.lua

@@ -2,23 +2,6 @@
 --
 -- @module fs
 
-function list(path) end
-function combine(base, child) end
-function getName(path) end
-function getSize(path) end
-function exists(path) end
-function isDir(path) end
-function isReadOnly(path) end
-function makeDir(path) end
-function move(from, to) end
-function copy(from, to) end
-function delete(path) end
-function open(path, mode) end
-function getDrive(path) end
-function getFreeSpace(path) end
-function find(pattern) end
-function getDir(path) end
-
 --- Returns true if a path is mounted to the parent filesystem.
 --
 -- The root filesystem "/" is considered a mount, along with disk folders and
@@ -31,33 +14,6 @@ function getDir(path) end
 -- @see getDrive
 function isDriveRoot(path) end
 
---- Get the capacity of the drive at the given path.
---
--- This may be used in conjunction with @{getFreeSpace} to determine what
--- percentage of this drive has been used.
---
--- @tparam string path The path of the drive to get.
--- @treturn number This drive's capacity. This will be 0 for "read-only" drives,
--- such as the ROM or treasure disks.
-function getCapacity(path) end
-
---- Get attributes about a specific file or folder.
---
--- The returned attributes table contains information about the size of the
--- file, whether it is a directory, and when it was created and last modified.
---
--- The creation and modification times are given as the number of milliseconds
--- since the UNIX epoch. This may be given to @{os.date} in order to convert it
--- to more usable form.
---
--- @tparam string path The path to get attributes for.
--- @treturn { size = number, isDir = boolean, created = number, modified = number }
--- The resulting attributes.
--- @throws If the path does not exist.
--- @see getSize If you only care about the file's size.
--- @see isDir If you only care whether a path is a directory or not.
-function attributes(path) end
-
 -- Defined in bios.lua
 function complete(sPath, sLocation, bIncludeFiles, bIncludeDirs) end