Update dependency less to v3.10.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
less (source)	devDependencies	minor	3.9.0 -> 3.10.0

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Release Notes

less/less.js

### [`v3.10.0`](https://togithub.com/less/less.js/blob/master/CHANGELOG.md#​3100-beta) [Compare Source](https://togithub.com/less/less.js/compare/v3.9.0...v3.10.0) 2019-08-03 - Switched codebase to ES2015 w/ Rollup/Babel compilation - Added TS linter to ESLint (types yet to be added to functions / vars) - Ignore undefined content in generating source maps ([#​3360](https://togithub.com/less/less.js/issues/3360))

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