Fix the fallback path to the l10n folder (#1104) (#1105)

If the language server client doesn't specify the location of the l10n
folder, then the language server falls back to a path relative to the
complied file.
This path was wrong; `server.js` is in `./out/server/src/server.js`,
so the relative path of the `l10n` folder is `../../../l10n` instead of
`../l10n`.

Signed-off-by: David Thompson <[email protected]>
Co-authored-by: David Thompson <[email protected]>

Author: msivasubramaniaan