Make tree-sitter-explorer support selecting local parser

Make tree-sitter-explorer support selecting local parser

Now treesit-explore-mode will prompt the user to select a parser
to explore, rather than a language.  It'll also include the
local parsers at point for selection.

* lisp/treesit.el (treesit--explorer-language): Change to
treesit--explorer-parser.
(treesit--explorer--nodes-to-highlight):
(treesit--explorer-refresh): Change to use parser.
(treesit--explorer-generate-parser-alist): New function.
(treesit-explorer-switch-parser): New command.
(treesit-explore-mode): Use switch-parser to setup.
* doc/lispref/parsing.texi (Language Grammar): Mention
treesit-explorer-switch-parser.

(cherry picked from commit 10b4d3045e1b856009c5ac1e1a1ca257f2d4493f)