v0.8.7

• Lexer bug fix: Only register elif/else/fi as keywords if they're
  lvalues
• Build system: Simplified and generalized build system with
  auto-discovered sources
  Refactored the build system to remove per- directory object rules
  and replace them with a single generic pattern that mirrors the
  source tree into bin/obj.
• Code style improvements:
  Replaced global bpp_exit_code with data member of the Listener class
  Replaced pointer arithmetic in parse_arguments() with std::span usage
  Added default case to xgetopt switch
  No need to explicitly default virtual destructors for LSP message
  base types
  Explicit member initialization everywhere
  Use emplace_back() for temporaries
  Explicitly handle ownership semantics in BashppParser
  Use BashppParser.setInputFromFilePath instead of from FilePtr where
  ownership may be ambiguous
  bpp-lsp: Moved URI validation to freestanding function
  Option parsing: Allow non-regular files (such as /dev/null) for output,
  just check whether we have write permission
• Test suite: Added '-c' option to benchmark compile times without actually
  running the compiled tests
• Parser: error out of datamember declarations when given an lvalue object
  reference instead of a proper object instantiation