Rename PostCSS files to .pcss (#9013)

* Rename PostCSS files to `.pcss`

* Make Stylelint happy

* Delint

* Rename new files too

* delint

* Fix bad comment placement

res/css/_font-sizes.pcss

@@ -0,0 +1,73 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+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
+
+    http://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.
+*/
+
+$font-1px: 0.1rem;
+$font-1-5px: 0.15rem;
+$font-2px: 0.2rem;
+$font-3px: 0.3rem;
+$font-4px: 0.4rem;
+$font-5px: 0.5rem;
+$font-6px: 0.6rem;
+$font-7px: 0.7rem;
+$font-8px: 0.8rem;
+$font-9px: 0.9rem;
+$font-10px: 1.0rem;
+$font-10-4px: 1.04rem;
+$font-11px: 1.1rem;
+$font-12px: 1.2rem;
+$font-13px: 1.3rem;
+$font-14px: 1.4rem;
+$font-15px: 1.5rem;
+$font-16px: 1.6rem;
+$font-17px: 1.7rem;
+$font-18px: 1.8rem;
+$font-19px: 1.9rem;
+$font-20px: 2.0rem;
+$font-21px: 2.1rem;
+$font-22px: 2.2rem;
+$font-23px: 2.3rem;
+$font-24px: 2.4rem;
+$font-25px: 2.5rem;
+$font-26px: 2.6rem;
+$font-27px: 2.7rem;
+$font-28px: 2.8rem;
+$font-29px: 2.9rem;
+$font-30px: 3.0rem;
+$font-31px: 3.1rem;
+$font-32px: 3.2rem;
+$font-33px: 3.3rem;
+$font-34px: 3.4rem;
+$font-35px: 3.5rem;
+$font-36px: 3.6rem;
+$font-37px: 3.7rem;
+$font-38px: 3.8rem;
+$font-39px: 3.9rem;
+$font-40px: 4.0rem;
+$font-41px: 4.1rem;
+$font-42px: 4.2rem;
+$font-43px: 4.3rem;
+$font-44px: 4.4rem;
+$font-45px: 4.5rem;
+$font-46px: 4.6rem;
+$font-47px: 4.7rem;
+$font-48px: 4.8rem;
+$font-49px: 4.9rem;
+$font-50px: 5.0rem;
+$font-51px: 5.1rem;
+$font-52px: 5.2rem;
+$font-78px: 7.8rem;
+$font-88px: 8.8rem;
+$font-400px: 40rem;