Improved Language Specific Keycodes for US International and Extended Layouts

[spidey3]

Description

This pull request improves a corner of the Language Specific Keycodes:

• Improvement for the US International keyboard layout [aka xkb:us:intl:eng - English (US, international with dead keys)]
  • Support for more symbols added
  • Fixed some confusion between dead-key accents and standalone counterparts (US_DGRV vs. US_GRV, US_DTIL vs. US_TILD, etc.)
  • Adds sendstring_us_international.h that will (if included) make send_char() / send_string() work correctly when the host is configured for this keyboard
• Adds support for US Extended keyboard layout [aka xkb:us:altgr-intl:eng - English (international AltGr dead keys)]
• Adds support in send_char() / send_string() for ASCII characters that can only be entered by typing a dead key followed by a space.

I've targeted this at the develop branch, because:

• The change to send_char() increases firmware size slightly (by about 62 bytes) for all keyboards.
• The changes in keymap_us_international.h make it more correct, but may break keymaps that depend on it (although I note, there doesn't seem to be anything in the qmk/qmk_firmware tree that uses it at present).

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[spidey3]

@fauxpark, et. al.:
I've fleshed this out a lot more, resolving the following issues

• Made send_string() work when only way to get an ASCII character is using a dead key. Note this costs ~60 bytes for all keyboards.
• Separated the linux variant from the base version of US International. The Linux variant builds on the base version; it seemed inelegant to make an entirely new map that differed only for a few keys.
• Addressed the ACUT vs. NDAC vs. DACU confusion. Bottom line: DACU is Double Acute; NDAC is non-dead Acute.

[fauxpark]

Thanks!