docs: update documentation

Signed-off-by: ldelossa <louis.delos@gmail.com>

perf: async symbol gathering

LSPs may return different results depending on the method being
requested.

for instance gopls will return no receiver name when a call hierarchy
request is made, but will when a workspace symbol request is made.

to work around this we make a workspace symbol request for each call
hierarchy item returned when a calltree is invoked.

this proved to be very slow when done sync so this commit makes it
async using lua coroutines.

Signed-off-by: ldelossa <louis.delos@gmail.com>

ui,tree: use workspace symbol if possible

after researching ldelossa#10 a bit more I found that certain LSPs (gopls here)
provides more accurate symbol information via a workspace symbol query.

it would be nice to receieve the same symbol info regardless of the lsp
method being used. see: golang/go#49690

this pr now resolves the workspace symbol for all call hierarchy items
placed into the tree.

this is a bit slower (with gopls anyway) on start, however I've found
that after some caching the results are snappy.

Signed-off-by: ldelossa <louis.delos@gmail.com>

ui: improved hover popup

this commit creates a custom hover handler which floats above all other
windows when trigged from the calltree ui.

Signed-off-by: ldelossa <louis.delos@gmail.com>

docs: update docs for highlighting

this commit adds documentation explaining highlighting and how to
configure it.

Signed-off-by: ldelossa <louis.delos@gmail.com>

v0.0.1: initial implementation

this commit is the v0.0.1 initial implementation of calltree.

Signed-off-by: ldelossa <louis.delos@gmail.com>