Error: spawn slither-lsp ENOENT (PATH issue)

Hey, just bringing it up to your attention.

[This PR is still relevant](https://github.com/crytic/contract-explorer/pull/45). Extension has problem finding [slither-lsp](https://github.com/crytic/contract-explorer/blob/789dc17aedab7268e2719b048fa751c0fed509d1/src/slitherLanguageClient.ts#L18), even if `echo $PATH` lists appropriate directories (and, `slither-lsp` is installed). This happens for both global and venv active python interpreter.

The only way to run extension, for me, was to do the [dev-installation](https://github.com/crytic/contract-explorer/blob/master/DEV.md) and point to the specific slither-lsp server running. However, even with that, the currently released version seems to be missing some functionality. Below is the only visible display. Plugin doesn't seem to do anything else than just allowing to toggle options for detection filters.


Start with src/slitherLanguageClient.ts at the referenced line and compare the behavior with PR 45. Review DEV.md and test how the released extension locates slither-lsp under global and virtual-environment Python setups; done means the extension starts with an installed server and exposes the expected functionality.

Written by the indexing model from the issue text.