Emit enclosing ranges for declarations

[varungandhi-src]

Based on #293, but created a separate PR since I couldn't push
to that branch. The following declarations have enclosing ranges:

• Named arrow function declaration
• Function declaration
• Class declaration
• Method declaration
• TS Interface declaration
• Enum declarations (not present in Implement enclosing ranges #293)
• Type alias declarations (not present in Implement enclosing ranges #293)

TODOs

• Do some basic benchmarking to see if this has a noticeable effect on runtime. If so, we should put it behind a feature flag. -- Measured against sourcegraph/sourcegraph, no noticeable difference in perf averaged over 10 runs.

Test plan

Covered by newly added snapshot tests.