Add completion support for cabal files

Currently supports:
* completions for keywords, sensitive to stanzas
* value completions for constant values, licenses, files and directories

Author: Jana Chadt

plugins/hls-cabal-plugin/hls-cabal-plugin.cabal

@@ -27,6 +27,7 @@ library
   exposed-modules:
     Ide.Plugin.Cabal
     Ide.Plugin.Cabal.Diagnostics
+    Ide.Plugin.Cabal.Completions
     Ide.Plugin.Cabal.LicenseSuggest
     Ide.Plugin.Cabal.Parse
 
@@ -45,8 +46,10 @@ library
     -- This is a lot of work for almost zero benefit, so we just allow more versions here
     -- and we eventually completely drop support for building HLS with stack.
     , Cabal                 ^>=3.2 || ^>=3.4 || ^>=3.6 || ^>= 3.8 || ^>= 3.10
+    , containers
     , deepseq
     , directory
+    , filepath
     , extra                 >=1.7.4
     , ghcide                == 2.0.0.0
     , hashable
@@ -57,6 +60,7 @@ library
     , regex-tdfa            ^>=1.3.1
     , stm
     , text
+    , text-rope
     , unordered-containers  >=0.2.10.0
 
   hs-source-dirs:   src
@@ -71,11 +75,14 @@ test-suite tests
   build-depends:
     , base
     , bytestring
+    , directory
     , filepath
     , ghcide
     , hls-cabal-plugin
     , hls-test-utils    == 2.0.0.0
     , lens
+    , lsp
     , lsp-types
     , tasty-hunit
     , text
+