Install CMake package config files with Meson, and add Meson options

Squashed onto develop from:
- Install CMake package config files with Meson
- meson: Indent code inside an if block
- meson: set a minimum Meson version
- meson: use `override_dependency()` to set dependencies
- meson: use `install_subdir` for headers
- meson: set the C++ standard to C++11
- meson: handle single header and multiheader the same way CMake does
- meson: add support for the GlobalUDLs option
- meson: add support for the ImplictConversions option
- Add meson information to FILES.md
- Complete the Meson options and match CMake's compile definitions
- Add the JSON_* definitions to the CMake pkg-config file
- Document the Meson options and check them in CI
- Fix the Meson CMake target for includedir or datadir outside the prefix
- Avoid //include in Meson-generated CMake target for prefix /
- Add DisableTupleReferenceConversion to Meson and the CMake pkg-config file
- Check in CI that Meson and pkg-config offer the CMake options
- Remove accidentally committed Python bytecode

Co-authored-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

meson_options.txt

@@ -0,0 +1,54 @@
+option(
+    'MultipleHeaders',
+    type: 'boolean',
+    value: false,
+    description: 'Use non-amalgamated version of the library',
+)
+option(
+    'GlobalUDLs',
+    type: 'boolean',
+    value: true,
+    description: 'Place user-defined string literals in the global namespace',
+)
+option(
+    'ImplicitConversions',
+    type: 'boolean',
+    value: true,
+    description: 'Enable implicit conversions',
+)
+option(
+    'DisableEnumSerialization',
+    type: 'boolean',
+    value: false,
+    description: 'Disable default integer enum serialization',
+)
+option(
+    'DisableTupleReferenceConversion',
+    type: 'boolean',
+    value: false,
+    description: 'Disable conversion from a one-element tuple of a JSON reference',
+)
+option(
+    'Diagnostics',
+    type: 'boolean',
+    value: false,
+    description: 'Use extended diagnostic messages',
+)
+option(
+    'Diagnostic_Positions',
+    type: 'boolean',
+    value: false,
+    description: 'Enable diagnostic positions',
+)
+option(
+    'LegacyDiscardedValueComparison',
+    type: 'boolean',
+    value: false,
+    description: 'Enable legacy discarded value comparison',
+)
+option(
+    'StrictNulHandling',
+    type: 'boolean',
+    value: false,
+    description: 'Enable strict NUL-byte handling',
+)