Use license rather than license-file

[michel-slm]

With license-file, crates.io shows the license as non-standard: https://crates.io/crates/websteer. Declare the license as per SPDX: https://spdx.org/licenses/

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This change is 

[michel-slm]

ps I've been planning to work on a similar tool for a while - and use it as an opportunity to learn Rust -- and didn't get round to it until after you published this. Didn't realize it exists until I was touching up my Cargo.toml prior to publishing and want to see if using browser as a keyword makes sense!

Mine is much more bare-bones than yours (and uses toml as the config file). Not even using regex yet, just substring matches -- but anyway, here it is for comparison. https://crates.io/crates/choosier