Add missing css.types.length.em feature

[queengooborg]

This PR is a part of a project to add missing interfaces and interface features to BCD that are from an active spec (including WICG specs) and is supported in at least one browser. This particular PR adds the missing em member of the length CSS value type. This mirrors the data from rem (which is probably accurate enough for our needs). Fixes #15293.

[kepstin]

The browser compatibility data included in this PR is definitely not correct. To be honest, I don't really think that tracking this unit's support is meaningful, except for people interested in history of 1990s browsers. It was part of the initial CSS 1 specification after all.

From a mix of some historical digging and personal experience, the em unit would have been available starting with Internet Explorer 3, Netscape 4.5 (although its use of a CSS to JSSS translator made it horribly buggy to use any CSS at all), and Opera 3.5.

Support for the em unit in the KHTML engine predates the fork into WebKit that produced Safari and Chrome. Support for the em unit in the Gecko engine was present from the start, predating the first stable release of the Mozilla suite and the start of the Phoenix/Firebird/Firefox project.

For the purposes of the compatibility data tracked here, all browsers should have support set to the first known browser release, except for Opera (use 3.5) and IE (use 3).

[Elchi3]

Thanks, sometimes it is useful to know which features are around from the start as new web developers don't necessarily have the knowledge how web platform technologies evolved over time.