Add basic Ngspice syntax highlighting

[gszymaszek]

​Ngspice is the open source spice simulator for electric and electronic circuits.

The attached patch adds basic support for highlighting Ngspice/SPICE circuit description files syntax. Two main contexts are defined:

• the default context, where the circuit is described,
• the control script context (between .control and .endc).

The default context highlights dot commands, circuit element instance names and model types. The control script context highlights supported functions/commands and known variables. Both contexts highlight comments and line continuation marks.

Other than two FIXMEs, it is a known issue the first line of the file is part of the default context, while in most cases it would contain the file title.

[andrew_b]

Thanks!

Applied as [52d9d7946f8c26ebb99dbf60834e7dc72b65957c].