feat: regex substitutions

[loeffel-io]

supports regex substitutions

every directory can be referenced by ${i}
for instance, directory test/google/linter will expose 3 variables:

• test: ${2}
• google: ${1}
• linter: ${0}

example:

ls: 
    components/*: # e.g.: components/button
        .ts: regex:${0}  # e.g.: components/button/button.ts
        .*.scss: regex:${0} # e.g.: components/button/button.module.scss
        
        tests:
            .test.ts: regex:${1} # e.g.: components/button/tests/button.test.ts

future plans

• modifiers like ${0:kebab-case}
• performance optimizations: substitutions can be cached etc

closes #59