The transpiler itself is fpc, the package you found only the 'sugar' for Lazarus.
No, it's not.
pas2js is contained in a separate repository, it only uses code that's part of FPC (namely
fcl-passrc).
Also the pas2js related packages for the RTL (or anything else) must not be
installed in Lazarus, because then Lazarus will try to compile them with FPC which is wrong. One simply needs to open e.g. the
pas2js_rtl package once and then add it as a requirement to a Pas2JS project (which requires one package to be installed:
Pas2JSDsgn which is provided together with Lazarus).