Fonts used for in-car display must have specific inter-word spacing

[simoncozens]

(related to #1832)

Observed behaviour

According to section 4.5.5 of ISO 15008 (the standard which governs the use of in-car displays), fonts used in this environment should "be evenly and proportionally spaced". Specifically, the word space should be between 250% and 300% of the lm space.

fontbakery should check for this.

Expected behaviour

There should be a check in a (new) iso15008 profile, with:

• an ID of com.google.fonts/check/iso15008_interword_spacing
• the following rationale:

According to ISO 15008, fonts used for in-car displays should be evenly and proportionally spaced.
To ensure legibility of this font on in-car information systems, it is recommended that the space character should have advance width between 250% and 300% of the space between the letters l and m.
(Note that PASSing this check does not guarantee compliance with ISO 15008.)