feature: add command-line completions

[Jisu-Woniu]

Trying to add command-line completions.

Closes #130

[Jisu-Woniu]

There is an issue when building if I try to generate completions in build.rs:

The LONG_VERSION variable here:

https://github.com/Jisu-Woniu/typstyle/blob/43ead3fb62c635c844b26a51265d1297b73b137b/src/cli.rs#L10

https://github.com/Jisu-Woniu/typstyle/blob/43ead3fb62c635c844b26a51265d1297b73b137b/src/cli.rs#L64

Is not accessible since VERGEN_CARGO_TARGET_TRIPLE is not available during build.

[Jisu-Woniu]

The code now compiles, but cli.rs seems quite ugly now...

[Enter-tainer]

I will review this pr when i am back from vacation (20th)

[Enter-tainer]

I will look into it this weekend

[Enter-tainer]

overall LGTM.

[Enter-tainer]

thank you!

[Jisu-Woniu]

There is a problem when I revert the changes, the generated completions are using the wrong binary name.

[Jisu-Woniu]

For example, if I run cargo run -- completions bash, what I get is:

_target/debug/typstyle() {
    local i cur prev opts cmd
    COMPREPLY=()
    cur="${COMP_WORDS[COMP_CWORD]}"
    prev="${COMP_WORDS[COMP_CWORD-1]}"
    cmd=""
    opts=""

    for i in ${COMP_WORDS[@]}
    # ...

which is expected to be:

_typstyle() {
    # ...