Add to FAQ how to profile Vetur

[octref]

For issues like #1179.

• Add vetur.dev.vlsPort setting to running VLS on a specific port
• Include a doc for profiling language server

[gombosg]

I'm waiting for some useful instructions on how to do this, since I want to help track down this annoying high CPU use bug.

[octref]

[octref]

@gombosg You can follow https://github.com/vuejs/vetur/blob/master/.github/PERF_ISSUE.md to report new performance issues.