Parse JSON false as :json-false instead of nil

This would break code actions where the server sends us an action
containing a `false` boolean parameter. `lsp-mode` would translate this
into `nil`, and then send a `null` back to the language server. An
example of this causing problems was
emacs-lsp#4184.

There may be parts of `lsp-mode` that expect `false` values to be parsed
as `nil`, so any logic involving parsed booleans may need to be updated.

lsp-mode.el

@@ -6740,7 +6740,7 @@ server. WORKSPACE is the active workspace."
  'plist
  'hash-table)
  :null-object nil
- :false-object nil)
+ :false-object :json-false)
  `(let ((json-array-type 'vector)
  (json-object-type (if lsp-use-plists
  'plist
@@ -6757,7 +6757,7 @@ server. WORKSPACE is the active workspace."
  'plist
  'hash-table)
  :null-object nil
- :false-object nil)
+ :false-object :json-false)
  `(let ((json-array-type 'vector)
  (json-object-type (if lsp-use-plists
  'plist
@@ -8626,7 +8626,7 @@ When ALL is t, erase all log buffers of the running session."
  (done (lsp--handle-process-exit workspace ""))))))
  :object-type object-type
  :null-object nil
- :false-object nil))
+ :false-object :json-false))
  "*json-rpc-connection*"))
  (cons con con)))