Option to specify path to rebar3 in vscode extension

[weaversam8]

Some of my projects use a rebar3 file committed to the repository, to ensure all developers use the same version. This means our invocations to rebar become ./rebar3 instead of relying on rebar3 being on the path.

The VS code extension enters into a degraded mode if rebar3 is not on the path. The extension should provide a config option that allows the user to specify the path to rebar3.

[michalmuskala]

Makes sense - it probably would be better to set this in the elp.toml config file, rather than in VS Code - since then the CLI would also work correctly

[robertoaloi]

@weaversam8 How would you feel about contributing?