[enhancement] cache tsconfig in server to avoid lookups during linting

[spence-s]

We implement the tsconfig lookup here and improve out-of-the-box performance when parserOptions.project is not set by the user. User's who set parserOptions.project will have done this anyway.

• automatically lookup and cache the tsconfig for a file when the file is opened
• destroy the cache when file is closed

This should improve the linting speed when no parserOptions.project is set.