[mypyc] Refactor: reuse format string parser (#10894)

`parse_conversion_specifiers` and `parse_format_value` from mypy.checkstrformat 
can be reused when compiling string formatting. `can_optimize_format` and 
`split_braces` are useless now.

Adding `start_pos` as an attribute of ConversionSpecifier can not only help parse 
literals but also help generate useful error messages in the future.

Author: 97littleleaf11