parser: do not crash on tokens without location

Do not insert them in the linked lis of tokens
Do not use them to compute locations on productions

Signed-off-by: Jean Privat <jean@pryen.org>
parser_util: drop injected tokens

Do not locate them so they are excluded by the parser.
Remove parents in the result of parse_something so injected tokens
become unaccesible.

Signed-off-by: Jean Privat <jean@pryen.org>
astutil: do not crash if the AST is just a token

Signed-off-by: Jean Privat <jean@pryen.org>
tests: update test_markdown because the new highligther

Signed-off-by: Jean Privat <jean@pryen.org>
parser_util: parse_somethig behave correclty when the first token is an error

Signed-off-by: Jean Privat <jean@pryen.org>