Fix detection of R languageserver

[krassowski]

References

Fixes #97

Code changes

• add is_installed() for ShellSpec
• remove R script helper, use in-line command; allows to add specs for potential forks of languagesever easily (and test easily)
• adapt is_installed_args for Rlanguageserver spec
• do not start the R language server in debug mode by default any longer (reduces clutter in the console) as it works reliably well recently
• fix links in the docs to JupyterLab docs
• explain what we mean by shell token in the docs

User-facing changes

R language server will be correctly detected as absent if not installed in the current R environment.

Backwards-incompatible changes

None

Chores

• linted
• tested
• documented
• changelog entry