Switch from pico-args to lexopt

[taiki-e]

If there are multiple flags with values, it is impossible to parse the arguments correctly using the current pico-args.
For example, the following command will be parsed as --flag1 value1 and --flag2 value2 when parsed --flag1 first, which is not what we expect.

app --flag2 --flag1 value1 value2

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci