Ensure treesitter parse tree is up-to-date to support dynamic update

[wookayin]

Whenever get_location() is called, we request the treesitter parser
to refresh the parse tree for the current buffer up to date.

This may be not needed when the treesitter-highlight module is enabled,
but when the module is disabled the plugin may not display the
correct context information (as reported in #69).

Such calls will be throttled properly: refreshing the treesitter
parse tree will happen only when the buffer has changed.

[SmiteshP]

What counts as buf_tick? If the parser is made to parser at every keystroke in insert mode then that could be excessive

[SmiteshP]

Oh, nevermind. The function won't be called in insert mode anyways cause get_location will return the cached value. So this will effectively work on InsertLeave. Great, its much cleaner than setting up a autocommand too

[wookayin]

Thanks!

FYI, actually with the treesitter highlight plugin parser:parse() call will be made almost at every keystroke (like as frequent as TextChanged). Practically this implementation would call as frequent as InsertLeave (which is very reasonable). I tested and it indeed doesn't get called while we are in the insert mode. It also works with some commands that doesn't involve the insert mode, such as yy or dd motions.