Add .editorconfig file (#864)

This adds settings based on some common-sense values and the TensorFlow
Quantum project's current conventions, such as line length and use of
spaces instead of tabs. The settings match the corresponding ones in
`.pylintrc`.

Author: mhucka

.editorconfig

@@ -0,0 +1,54 @@
+# Copyright 2025 The TensorFlow Quantum Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+# Common editor configurations for this project.
+#
+# EditorConfig is a file format for specifying some common style parameters.
+# Many IDEs & editors read .editorconfig files, either natively or via plugins.
+# We mostly follow Google's style guides (https://google.github.io/styleguide/)
+# with only a few deviations for line length and indentation in some files.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+root = true
+
+# IMPORTANT: some of the other config files (.markdownlintrc, .jsonlintrc.yaml,
+# etc.) also have settings for indent and line length. When making changes to
+# this file, make sure to update the other files to match.
+
+[*]
+charset = utf-8
+indent_style = space
+insert_final_newline = true
+spelling_language = en-US
+trim_trailing_whitespace = true
+max_line_length = 80
+
+[{BUILD,*.BUILD,*.bzl,*.bazel,.bazelrc}]
+indent_size = 4
+
+[{*.cc,*.h}]
+indent_size = 2
+
+[*.json]
+indent_size = 2
+
+[*.py]
+indent_size = 4
+
+[*.sh]
+indent_size = 4
+
+[*.yml,*.yaml]
+indent_size = 2