this commit changes the `tsserver.js` resolution logic to use an
absolute path rather than a relative one. previously, if a user wanted
to use a `tsserver.js` that was local to a project but didn't live in
`node_modules` (which would be automatically resolved by default), the
`tsserver.tsdk` needed to be hardcoded to an absolute path. this
required the user to modify `coc-settings.json` each time they wished to
switch projects. by resolving to an absolute path, a user can now set
the value to a relative path which will resolve correctly per project.
this change facilitates simplified usage of Yarn v2's PnP feature, which
can generate a project-specific `tsserver.js` to enable correct IDE
integration.
