Add implementation for BSP workspace/reload

[Gedochao]

The reload generally works now, but there are some limitations on IntelliJ:

• if there are sources directly in the primary workspace directory (first input arg passed to the scala-cli setup-ide command), they will not be included in the new build after the reload if the project hash has changed.
• The reason for this is the old problem with intellij-scala creating an empty *-root module. Even though we work around it on BSP initialization (as per changes in Adjust BSP project base directory to scala-cli content root to fix IntelliJ import #780), it becomes harder when handling a workspace/reload request with changed sources. If the source directories change on reload, the build target ids also change (the bsp project has a new hash). As a result, some targets need to be deleted, while others are created and intellij-scala creates the *-root module in between, as it's their hack for there always being a module with the primary workspace source directory.

• Now, this is tolerable if the sources are hidden in subdirectories (screenshot above), as then this only causes an empty, harmless module to be added to the IntelliJ project structure. It gets worse if the workspace root directory also contains sources, as then we have a duplicate, and IntelliJ immediately gets rid of it just as we reload. This is because IntelliJ never allows for 2 modules to have overlapping source directories (and in all likelihood, the pre-reload build & the post-reload build share the root workspace directory). Thus, even though we reload correctly, we miss any sources in the root directory. (screenshot below)

• There may be some workaround for this, but I think it should be handled outside of the scope of this PR. Currently if a user runs into this, they have to either fix the project structure manually (by adding the correct sources directory to the BSP module and getting rid of the *-root module) or get rid of the .idea directory and reimport the project. Quite annoying 😞
• This issue is absent from Metals, of course.

[Gedochao]

It isn't a lot of overhead, but the args are effectively validated twice here (once for the IdeInputs instance, and once for the Inputs instance). I intend to try and refactor this in a future PR.

[Gedochao]

even though we return a valid error response to the workspace/reload request, it doesn't seem like IntelliJ is handling it properly, as no error message is shown in the UI. We may consider sending a BSP logMessage request, so the IntelliJ user has at least a slight chance of noticing that something went wrong (it'd show in the BSP build log, not just the BSP logs in the IntelliJ directory). showMessage isn't an option either, as IntelliJ ignores that as well. TBD, I'm open to suggestions (although I'd rather do it in a separate PR as well)

[Gedochao]

We're going ahead with a reworked version of this in #858, closing this.