In preparation for later changes to the workspace Symbol method, we add
a separate configuration option keyed by "symbolMatcher" that specifies
the type of matcher to use for workspace symbol requests. We also define
a new type SymbolMatcher, the type of this new option. We require
SymbolMatcher to be a separate type from Matcher because a later CL adds
a type of symbol matcher that does not make sense in the context of
other uses of Matcher, e.g. completion.
Change-Id: Icde7d270b9efb64444f35675a8d0b48ad3b8b3dd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/228122
Reviewed-by: Robert Findley <rfindley@google.com>