Make the xml output better match files in the sim/viewer

[tapple]

I made 4 changes that generate smaller diffs when compared against keywords_lsl_default.xml in the viewer and viewer cache. I needed that in secondlife/lsl-definitions#38 where I auto-generate those files

• Adds option c-compat to xml generation.
  • before: <boolean>true</boolean>. after: <boolean>1</boolean>
  • before: <real>10.0</real>. after: <real>10</real>
  • before: <real>0.1</real>. after: <real>0.1000000000000000055511151</real>
• Adds option sort_maps that sorts all the keys of a map alphabetically
  • I ended up not using this
• When the pretty-printer is used, empty strings and arrays are now collapsed:
  • before: <string></string>. after: <string />
  • before: <array></array>. after: <after />
• The first tag after <llsd> is now indented correctly when pretty-printing

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Part of a set of PR's improving the definitions and keywords files in lsl and slua:

• Add lua_definitions.yaml lsl-definitions#38
• Type-fixes viewer#5288
  • Updates and improvements to keywords_lua_default.xml viewer#5301
• Luau type fixes for luau-lsp sl-vscode-plugin#57
• Make the xml output better match files in the sim/viewer #37