Update tagline to mention lexical handlers

[jiribenes]

Found all occurences of the old tagline:

$ rg "A research language with effect handlers and lightweight effect polymorphism"

_config.yml
6:description: A research language with effect handlers and lightweight effect polymorphism

_layouts/default.html
9:    <meta name="description" content="A research language with effect handlers and lightweight effect polymorphism" />
17:    <meta name="og:description" content="A research language with effect handlers and lightweight effect polymorphism" />
21:    <meta name="twitter:description" content="A research language with effect handlers and lightweight effect polymorphism" />

_layouts/home.html
13:              A research language with effect handlers and lightweight effect polymorphism

and changed them to

A language with lexical effect handlers and lightweight effect polymorphism

[b-studios]

Thanks!