Add support for SGR 2 faint and SGR 9 strikethrough

[antonme]

This adds support for the ECMA-48 SGR attributes for faint (2) and
strikethrough (9), along with their reset sequences.

In particular:

• 2 enables faint / decreased intensity
• 9 enables strikethrough / crossed-out
• 22 clears both bold and faint
• 29 clears strikethrough

The implementation is intentionally small and only touches:

• src/terminal/terminalframebuffer.h
• src/terminal/terminalframebuffer.cc
• src/tests/emulation-attributes.test

It wires the already-defined faint attribute into the SGR parser/emitter and
adds strikethrough to the rendition enum so both attributes are parsed and
re-emitted correctly.

Test coverage extends the existing emulation-attributes regression test with
cases for:

• 2
• 9
• 1;22
• 2;22
• 9;29

This is a current rebase of the older #1059 change onto current master.