RATGen/force generator role handling updates

[SuperStucco]

The role handling for converting the force generator XMLs to RAT tables/generating random forces has a few holes that need patching, mostly around infantry roles and non-combat units. There were also a few errors, such as limiting MINELAYER/MINESWEEPER to Mechs (rather than ground vehicles) and infantry.

closes #5286

Role/unit type validation:

• adds missing SR_FIRE_SUPPORT and MIXED_ARTILLERY roles to unit type validations
• updates role/unit type validations to properly account for all unit type/role combinations
• organize and comment for readability (enum integer-equivalent less/greater than comparisons are pretty hard to follow at a glance unless you know all the values by heart)

Availability value modifications by role:

• adds missing roles for when specific roles are requested, mostly infantry but a few standard ones as well
• more extensive handling of non-combat roles (not needed now, but will be useful later)
• replace array index calls with named variables for readability
• relax application of role modifiers when no roles are called for, while still filtering out non-combat units
• organize and comment for readability

Non-combat/support unit identification:

• simplifies identification of non-combat support and civilian units

Role parser:

• adds missing MIXED_ARTILLERY role to string parser

Help file is updated to list the full set of roles, along with valid unit types and typical uses.

[IllianiCBT]

I'm not able to see any issues here. Glad to see some of the missing roles added and our coverage expanded.

[IllianiCBT]

Never mind, I got so excited I failed to notice the PR is still in Draft. >_>

[SuperStucco]

Just making sure there isn't any last minute changes prompted by a round of data changes, now ready for review.
This should fix #5286

[IllianiCBT]

LGTM (for real this time)

[IllianiCBT]

Just making sure there isn't any last minute changes prompted by a round of data changes, now ready for review. This should fix #5286

If you add 'closes #5286' somewhere in your PR description, GitHub will link the two PRs. :)