Implement $find() & @split_at()

examples/methods.ion

examples/methods.out

src/parser/shell_expand/mod.rs

src/parser/shell_expand/words/methods/arrays.rs

src/parser/shell_expand/words/methods/mod.rs

src/parser/shell_expand/words/methods/pattern.rs

src/parser/shell_expand/words/methods/strings.rs