Unused build depdency on regex

lsp-types/lsp-types.cabal contains regex in the build-depends for the generator executable. It is however never used. https://github.com/haskell/lsp/blob/master/lsp-types/generator/CodeGen.hs#L115 has `-- TODO: replace with regex` but that has not been implemented. regex depends on regex-pcre which depends on the end of life pcre1 library that Arch Linux wishes to remove. Would it be possible to remove the dependency? @felixonmars @dvzrv

Start in lsp-types/lsp-types.cabal and inspect the generator executable's build-depends. Check generator/CodeGen.hs around line 115 to confirm the regex TODO is not implemented, then remove the unused dependency and verify that the generator still builds successfully without it.

Written by the indexing model from the issue text.