Add extract_query function to retrieve query strings from URLs

[ghxm]

Since the package currently only includes a drop_query() function to handle queries, an extract_query() analogous to extract_path() could be used to extract the query string into its own column.

• Add extract_query() to extract the query string from URLs into a new column, complementing the existing extract_host(), extract_domain(), and extract_path() functions
• Uses adaR::ada_get_search() under the hood, follows the same signature pattern (varname, decode parameters) and column naming convention
• Add tests and mention in vignette/README

[chainsawriot]

@ghxm

• Add yourself to DESCRIPTION; and then run cffr::cff_write()
• run devtools::build_readme() and commit also README.md

[ghxm]

@ghxm

• Add yourself to DESCRIPTION; and then run cffr::cff_write()
• run devtools::build_readme() and commit also README.md

Added myself as a contributor and committed README.md changes. CITATION file does not change with contributor updates (other than date updates and dependency information), so I skipped that.