Add --filter and --reject options

[veeceey]

Adds two new flags for command-based result filtering:

• --filter <cmd> ; - runs a command for each match, keeps the result only if it exits 0
• --reject <cmd> ; - inverse of --filter, excludes results where the command exits 0

These use the same placeholder syntax as --exec ({}, {/}, {//}, {.}, {/.}).

Example usage:

# Find files containing "TODO"
fd -t f --filter grep -q TODO {} \;

# Find files that are NOT symlinks
fd --reject test -L {} \;

The commands run with stdout/stderr suppressed so they don't pollute the output. Filter/reject runs in the walker threads alongside the other built-in filters (size, time, type, etc.) so it integrates naturally with the existing pipeline.

Closes #400

[veeceey]

Test results (macOS, local build):

cargo test
test result: ok. 103 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Manual testing:

# Setup: src/exec/ has 3 files: command.rs, job.rs, mod.rs
# Only job.rs and mod.rs contain "CommandSet"

# --filter: keep files containing "CommandSet"
$ fd . src/exec/ -t f --filter grep -q 'CommandSet' \;
src/exec/job.rs
src/exec/mod.rs
# ✓ Correct - command.rs excluded

# --reject: exclude files containing "CommandSet"  
$ fd . src/exec/ -t f --reject grep -q 'CommandSet' \;
src/exec/command.rs
# ✓ Correct - only command.rs (which doesn't have CommandSet)