This commit changes `languagetree.lua` so that it creates a scratch
buffer under the hood when dealing with string parsers. This will make
it much easier to just use extmarks whenever we need to track injection
trees in `languagetree.lua`. This also allows us to remove the
`treesitter.c` code for parsing a string directly.
Note that the string parser's scratch buffer has `set noeol nofixeol` so
that the parsed source exactly matches the passed in string.
2a7cb32959
